Problems with Hero Impulse

Hero Impulse being the only dirt oriented motorcycle in the country which was produced and launched by Hero Motor Corp in 2012 and sadly was discontinued in the same year itself leaving us (the dirt biking enthusiasts of India) deserted. But the emotion lurking deep beneath is what has been driving us to several places on our only dirt bike Hero Impulse. But what Hero Impulse owners have witnessed and has been concerned a lot about is the impotency of the bike mainly the stock mill 150cc in displacement which is incapable of getting itself through several moments which one may encounter while travelling, riding on it over a long time. This precisely encircles tourers and north Indian biking enthusiasts riding on this 150cc dual sport machine. Riding overtime on this machine on the stock mill for almost 40000kms across north India through various passes with ladakh, spiti and several kms under that 40k mark on the ODO had me through all of what I could strive for in order to get the 150cc mill running in all conditions. But not to mention the awesome chassis and the suspensions and ride quality which is unmatchable in any other bike in India. So with few problems aside it’s a blessing to have a bike like Impulse in your garage but rectifying these problems has helped me and others to get us to places un-imagined in a way undo-able on any other bike as on Impulse. Thus I`m penning down few problems, their causes and remedies hoping to help the Impulse owners get that little extra which it lacks under certain conditions.

Problems:

Here are a few big problems listed below witnessed by me and my 7-8 fellow Impulse owners and lots of Impulse owners` rant over Facebook and automotive forums like Xbhp and BcmTouring. All witnessed the same problems at different times depending on kms run and time spent riding their respective Impulses.

1)      Altitude sickness

2)      Low fuel economy

3)      Engine RPMs stuck at 5/6000

4)      Slide needle breaking

5)      Noisy cone set

Altitude Sickness

CONDITION

Probably the biggest problem of the Hero Impulse and indeed a curse when you love touring and scaling high mountains and all what you’ve got is a Hero Impulse in your garage. But I love my dirt bike and I love taming terrains standing on the pegs throttling wide tearing the landscape in style but the stock mill robs you of it and it did so when I got mine out after the run-in to the nearby mountains. It wouldn't climb even to 8000ft ASL (above sea level). A mild climb to a normal hill station mere 23 kms away situated at 8000ft is easily done on a HH Splendor than on a Hero Impulse.

                Thus called altitude sickness in vehicles is the process where air gets thin for a machine (humans too) and the engine finds it difficult to breathe as we do. In the case of engine what happens is the air gets thin and the in turn decreasing the amount of oxygen entering the engine via Carburettor and disrupting the A/F (Air-fuel mixture) which was set at your home at an altitude level less as compared to that of where your mill starts to feel sick and start taking hiccups which is different prior to setting off for a tour or a road trip to high mountains which are at higher altitudes as compared to your home. So the A/F ratio changes as the altitude changes. 

              What happens in Impulse is that it starts taking hiccups as you reach around mere 8000ft ASL. Signs like slow acceleration and power lag tend to show up prior to hiccups climbing high altitudes. Reaching at 10000+ft what I witnessed was bike at 7000-8000rpms doing 30-35km/h on an all tarmac mild gradient coming to a stop when it refused to climb further at 1^st gear with throttle almost wide open. Fun out of the window now finding it hard to get the bike through the landscape one ends up with an overheated engine, running rich which has been strangled and ripped redlining on 1^st gear sluggishly to reach the top. And getting down the gradient in neutral is no fun and dangerous too especially when is off road given the hiccups and no power at high altitudes.

                Often shutting off the engine for a while and letting it cool down is the only option you`ve got and when you are finished pondering over the surrounding landscape (which is less done munching miles on a high speed tourer thus one of the boons riding a stock impulse) the mill runs fine for a few 50-100 metres getting back again to its miserable state only to improve once you loose on altitude and get down to lower areas. 

PROBABLE CAUSES & REMEDIES

                The main cause of the mill getting choked and impotent at even not so high altitudes is the way the stock 150cc mill is designed. Not getting too deep into the engine operational procedures, what causes this is are the following:

a)      The 28mm carburetor with main jet sized 132

b)      The catalytic convertor equipped exhaust

c)       Restrictive exhaust header

d)      The restrictive paper element type Air Filter

To put things straight, the main jet size on Hero Impulse is 132 which is darn big. Even the 200cc+ mills equipped the same Keihin carbs run on main jet sizes below 132. In fact the Hero Honda Karizma`s has a mainjet size of 125. And with a 28mm carb w.r.t. 24 & 22 mm carbs of that of same 150cc motorcycles it looks pretty big which thus creates less pressure differentials at the venturi and thus drawing power at high rpms giving the bike a nice late-mid and top end power (unlikely to be on a dirt bike). Which I think would have been done better on a smaller carb with a small main jet size. 

I tried 125 main jet of ZMA to high altitudes of 8000-8500ft ASL which gave me better results but no significant gain enough to get the bike going effortlessly. Changing the A/F mixtures on mountain tops didn’t helped either. All what was happening was the air entering was less and fuel more thus emitting out black smoke occasionally at high rpms (due to unburnt fuel caused by less oxygen in the thin air which disrupted the A/F mixture ratio).

At these times what ones gotto do is allow more air in as much as possible. What tourers over time have been doing riding to Ladakh is removing the air filter or punching few holes in it just to land somewhere near the Ideal A/F mixture ratio by increasing the air inflow which has been decreased by thin air. Thus in short what happens in the stock 150cc mill on Impulse at high altitude is that air entering the engine through the carb is less as compared to that of fuel thus causing the A/F to vary and bike starts taking hiccups. So either get in more air in by removing or punching holes in the filter which is a measure to get in more air, but removing the air filter (paper type as in Impulse) will cause lots of dirt and grime entering the engine which I don’t this is wise enough. So few modifications or DIYs are mentioned below:

1)      Foam filter: Remove the paper element type filter because it is very restrictive (paper being more restrictive allowing less air) and replace that with a foam type. One can go for a K&N (cotton gauze type) as its more free and less restrictive but will allow more dirt which personally I`m not content with thus landing in between opting for foam filter allowing in more air as compared to paper type, marginally increasing the air flow.

How I did this was I bought a foam filter of a Pulsar and cut and shaped it according to the stock filter`s casing and stuck and placed it there. Apart from functional characteristics advantages of foam filter is that it washable and reusable. This helps a lot with high altitude problem in Hero Impulse.

2)      Main jet: If you can fiddle around with several jet sizes, what you can do is start from low jet sizes 110 to increase up to 125 main jet size or get it done the opposite way but taking several spark plug readings and carb tuning over frequent times. What I did was I tried different main jets overtime form 126, 123 to 119 which I further found hard to source and then tried Mikuni jets with the help of Mikuni-Keihin conversion tables (easily available over net) and found a jet almost perfect of 118. Which runs fine but not at high altitudes as high as 15000-16000ft. Crossing high passes such as Tangalang-La and Baralacha-la wasn’t doable on these jets. I would rather advice to stick to jets of respective make i.e. keihin jets for Keihin and Mukini for Mikuni. Coversion tables are not very accurate and precise. I get my jets from AliExpress HERE

Also to mention the carb needle attached to the diaphragm slide isn’t adjustable. Thus no scope of adjusting the needle height accordingly (which can be done to drop the needle a little lower to reduce the fuel amount entering he engine from the jet) thus reducing the fuel amount a lil more to match the air and bring the carb more close to the actual the A/F mixture. What one can do in this case (I haven’t done it yet) is to shave off the diaphragm plastic washer screw (white colour) that holds the needle, to drop the needle accordingly. One can use a small flat file to do this. Half an mm would do enough I guess.

Other than that one can always chip in for the wire in carb method which can be done once you start climbing altitudes or get that done by a mechanic. For example going to Ladakh if one wishes to get his carb wired to eliminate the altitude sickness then get your carb wired form either Manali or Keylong form a local mechanic or learn it. Wire in carb is a simple and effective measure to rectify high altitude sickness in motorcycle. 

The wired jet which gets screwed then in the carb 

What this wire in carb jet would do is…reduce the jet orifice which will draw less fuel to accompany with the less air. Lots of people do it and the mechanics thus are pretty acquainted with this. So either changing the jet (that would require a lot of experimentation and constant observations and readings and regular high altitude checkups) or getting the carb jet wired (way simpler as compared to the former).

3)      Exhaust :  The stock header pipe being very restrictive and the end barrel having the Catalytic Converter creates a lot of back pressure causing the air inflow to hinder a little which needs to be cared of. What one can do is get a new header bend pipe fabricated and the cat removed. Get the exhaust as much free as possible or get a new aftermarket free flowing exhaust. This will let the bike breathe more freely now allowing in quick and efficient exit of the exhaust and new air to flow in more easily. This will come with an advantage of a deeper throaty sound from your Impulse which will attract a little attention which the timid sound of the stock exhaust doesn’t gives you.

Cat removed from the stock Impulse exhaust (more free exhaust and better sound too). I planted a sapling as a compensation keeping carbon sequestration in mind.

LOW FUEL ECONOMY

Low fuel economy has been a real pain in this ass when you`re riding a 150cc machine and you expect to run it for at least 50-55kms to a litre on a bike like Impulse. Nonetheless given the final gears sprocketing to be 17/50 big rear sprocket the kmpl figures still isn’t impressive as 35-40kmpl is what Impulse gives when not driven that rash. Kmpl figures came out to be less than a Karizma and with an Impulse with a Zma engine too. This according me happens partly because of the sprocketing and high working rpms resulting in more power strokes per min and mainly because of the 28mm carb and the whopping 132 main jet size. This gets improved once you get the carb wired (you will loose a lot of top end though the low end gets improved a little) and you lower the carb needle which draws less fuel in both the cases. Also the big 28mm carb plays a counterproductive role in reducing jet sizes or wire in carb as the carb geometry doesn't suffice the decrease in jet size and less fuel thus eventually a big jet in the first place. Full circle. 

You can go for a new carburetor for Hero Impulse other than the stock one. Pulsar carburetor on hero Impulse works fine. BS29 from Pulsar180 is good. So is Apache and Pulsar 200NS carburetor on Hero Impulse. Bigger the carburetor better the top end. Also jetting plays a big role. This is just a rule of thumb. You might wan to read Dellorto carb manuals for detailed stuff.  

STUCK RPMs

If you have witnessed that suddenly the bike isn’t gaining speed but the rpms are climbing up doing nothing and soon you are on 7-8000rpms with mere 50kmph on the odo in 5^th gear and roms aren’t going up that smooth and the engine feels as if something has stuck its probably the slide (the diaphragm slide in the carb which goes up & down in the carb barrel). This slide gets scored overtime and bruised form the sides running I the barrel that the polish on the slide wears of exposing the rough surface beneath which gets more rough on scoring to stuck one day. Also the culprits are the two triangular depressions (see the attached pic below) at the bottom of the slide which causes the slide to stuck from the bottom when the slide rests at the end of the barrel in throttle closed position.

Several people have opted for new slides which on 6-7000kms after (though depends a lot on riding styles) gets bruised and stuck again. New slides cost you around 550rs. Either you can chip in for that or do a permanent solution to it by taking a file and make as smooth as possible those two triangular depressions at the bottom. Rub a little with a soft grade Emory paper the slide too (used one) and install it back in. Also try rotating the diaphragm exactly opposite (approx) to free those triangular depressions from any barrel contact (see via the carb intake while doing this). 

A new slide costing 500 rs approx will solve the problem as a whole if one wishes to opt for. Honda Unicorn's and Stunner is a direct fit too.

Bruised/Scored diaphragm slide

SLIDE NEEDLE BREAKING

With 20000+ kms (usually happened in Impulses which ran above this mark) on the ODO what happens is the needle breaks and if you are lucky enough didn’t gets sucked into the engine as it did in mine snapping at Wide Open Throttle (WOT). Maybe the resonating vibrations is what causes this because it has always happened (twice though) when I`m open wide on throttle ripping the mill. Also the quality or the material of the slide is to be blamed here. Very fragile as it is as compared to other bikes is very prone to breakage. Happened twice, the part is hard to source thus I keep one in spare as it can happen anytime anywhere. The other time is happened it didn’t got sucked into the engine the broken part fell right into the carb jet sleeve, dismantled the carb to find the same there (lucky!). Carrying a spare needle is all what one can do because producing a new needle would be a high precision work which will require a lot of time and R&D etc.

CARB thus being the main source of all related problems until now I think was the poorly R&D-ed thing on the bike. It being 28mm on the first hand thus creating low pressure differentials and high throttle position (flat butterfly valve) consequently to extract fuel out of the 132 main jet (which has to be this big because of that) that vomits forth a lot of fuel results in low kmpl figures (lower than ZMA and Duke200) on a 150cc mill is preposterous and poorly worked upon. So a full carb change is also what you can opt for too. That being of other bikes or any aftermarket ones (that’s left to one`s own experience). Not to mention the other slide and needle related problems too.

Broken slide needle

Pressed needle bits once it got sucked in ! Damaging the valves too. 

CONE SET NOISES

Either cone sets or steering bearings or the steering races are what makes noises of sorts. What happens is over time the bearings get loose w.r.t. the races and they start to clickety-clack on potholes and rough patches. Overtime which gets more hideous and regular replacement of the cone set (600rs worth for a set). This happens due to the improper tightening of the steering races and bearings of the Impulse which is why half of the Impulse suffer with this problem half of them don’t leaving me suspicious of the assembly time. What needs to be done is to get the bearings and the races of a new cone set tightened properly (refer to youtube videos for the same) so they don’t come loose later. People have been replacing their cone sets at regular intervals every 7000kms approx. whereas the other owners haven’t replaced their cone set even once with 35000+kms on the ODO. So get the steering bearing installation procedures right (which might have been the case at the time of assembling). 

Broken bearing come loose when driven long on loose bearings and cones. 

Though a solution later came as Tapered Bearings which apparently solved these cone issues. These high duty bearings help keeping intact the races and bearings unlike the stock one. The tapered roller ones for the Impulse come with sizes 47mm(outside dia)x26mm(inside diameter). one can either get that of NBC or SKF both known well as leading bearing manufacturers. Installing these on few bikes have solved the bearing loosening issues and proved out potent enough to get through few rallies too. The one I got was NBC32005 bearing which measured 47x25mm thus I grinded out 1mm on the inner diameter to accommodate into the T stem of the bike which measures 26mm. I had to do this as there was no pin-precise taper bearing available for this measurement. 

26mm Inside dia w.r.t. T-Stem

Snug fit.

These are few basic problems which one may experience on Impulse riding it. This was a short guide for a noob that can help himself/herself getting to high passes and altitudes mainly and get his/her impulse running without any problems as such. 

If you wish to inquire more problems related to Hero Impulse which have gone unanswered you can ping me on Instagram or my Page HeroImpulse on which I have answered questions in detail in the highlights. 

 Power Hungry

Yes we are and who isn't when in comes down to riding Impulse. Every Impulse owner`s and criticizer`s lament comes out in the form of the interminable climbs and demeaning moments of touring times which the low displacement, bhp and torque figures are usually held accountable for. So with a lot of that and a little of what we thought might work, we happen to bump into a word quite synonymous with power hungry people called the BIG BORE. 

It wasn't long then a close friend had his sleeves up and came up with after a lot of R&D, error and revaluations a 195cc overbore block which sent strong vibes across the handle bars bars on the first crank marking its potency. The low end get a significant bump as the vehicle propels from rest beautifully equalling to a ZMA engine-d Impulse with those extra kgs off. The initial rpms are great on torque whereas the top end is almost the same where the rpms build up with a flat torque curve with respect to increasing rpms.

On a second note I am more of a tourer and I prefer to stay from too technical geeky automobile stuff and keep myself aloof from hassle and things like big bore. I prefer a simple Karizma engine swap with less intricacies and easy to mend character.

Now into the Break-In period

Contact me for more on Instagram HERE and download the Hero Impulse Parts Catalogue HERE