New exhausts poor running

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PM999S

PM999S

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While there are loads of threads on pilot/mixture settings, my problem doesn't seem to fit with the many very good suggestions. I recently installed new exhausts on my '72 Combat using baffleless peashooters. Initially, I jetted up to 280 from the 230s that were standard to compensate for the reduced back pressure and went for a quick test ride. The bike certainly revved up quicker although I was still unsure whether the jetting big enough albeit the plugs have never been cleaner. Partial closing of the throttle didn't markedly affect the acceleration so I thought things might still be a bit lean.
This is where things started to go wrong, because I decided to try out the 300 jets and change the aging filter with an unused K&N for the standard airbox from the 1970s that I found on eBay to see if it improved breathing even further. However, while the acceleration remained strong, closing the throttle set off coughing and spluttering to the point of popping and backfiring. So, I went back to the original settings complete with the old filter but the problem remained.
I then looked for air leaks and saw the previous owner had tried to repair the carb rubber hoses with a bike tyre repair kit. Certain that this was the issue, I fitted new hoses after carb-clean spraying the pilot/mixture channels and checking the seating of the floats. No discernable difference. Speeding up the idle and richening the mixture screws have helped a bit but not massively. I've also checked the seating of the new headers, secured by new brass roses and gaskets, and there is no movement there to let air in. What have I missed?
 
PM999S said:
I recently installed new exhausts on my '72 Combat using baffleless peashooters.
Click to expand...

Which peashooters (or other silencers/mufflers) were fitted beforehand as I would have expected them to also be the same baffleless type?

PM999S said:
Initially, I jetted up to 280 from the 230s that were standard to compensate for the reduced back pressure and went for a quick test ride.
Click to expand...

The main jets for a 750 Combat with standard (06-1978) peashooters would be 230* (260 for 850).

*Edit: 230 according to the factory manual (220 with mutes).
220 according to Amal's specifications.
 
Last edited:
I built my 850 to Combat specs, 2S cam, ports and shaved head work as well a open exhaust system, I run 270 main jets in my Amals, needles set on the bottom notch and with the Joe Hunt maggie my Hot motor runs great and has been set up this way for over 40 years now, I replace the needles and jets every 3 years as they do wear out from vibrations, make sure the manifolds aren't leaking or any air leaks and float levels are right, I have also ran with a K&N filter for the 40 years.
Because your plugs are running clean mean nothing really as its hard to judge plugs with todays fuels, maybe you have over jetted it, 300 main jets might be a bit high.
But of course mine is a hot 850 and not 750 as the Combats are and 300 jets might be just to much.
 
L.A.B. said:
Which peashooters (or other silencers/mufflers) were fitted beforehand as I would have expected them to also be the same baffleless type?



The main jets for a 750 Combat with standard (06-1978) peashooters would be 230 (260 for 850).
Click to expand...
I had silenced long-distance exhausts on before so changed up to peashooters without baffles in view of the fact I now live in a very rural location and the extra urge is nice. Having ridden it more I think the 280s with the K&N are about spot on on full chat, it's just the pesky closed throttle running that I'm struggling with.
 
ashman said:
I built my 850 to Combat specs, 2S cam, ports and shaved head work as well a open exhaust system, I run 270 main jets in my Amals, needles set on the bottom notch and with the Joe Hunt maggie my Hot motor runs great and has been set up this way for over 40 years now, I replace the needles and jets every 3 years as they do wear out from vibrations, make sure the manifolds aren't leaking or any air leaks and float levels are right, I have also ran with a K&N filter for the 40 years.
Because your plugs are running clean mean nothing really as its hard to judge plugs with todays fuels, maybe you have over jetted it, 300 main jets might be a bit high.
But of course mine is a hot 850 and not 750 as the Combats are and 300 jets might be just to much.
Click to expand...
I've got new manifold gaskets and manifold balance pipe, which looks a bit perished too, on order but these weren't touched between the first run with new pipes and jets that was fine on closed throttle and the other changes so unless something else has worn out, such as the needles just as a coincidence, I'm not sure if these will make things better.
 
FWIW 300 sounds very rich, jet-wise.

The mufflers not having baffles does not mean they produce more power; it just means they are louder. They may be worse as as far power depending on other factors of the design. I have personally seen LOUD straight - through "performance" car mufflers reduce power (on the dyno) compared to the fairly quiet factory mufflers.

I suggest you go back to the original jetting and compare the two mufflers performance before making any other change. Then fool with the jetting if it seems to be necessary. Unfortunately, doing this without a dyno or some objective means of gauging performance, makes it very difficult to determine the actual change, if any, in performance. The "louder is faster" phenomena, whether from louder exhaust or louder intake noise (or both) will fool most of the people most of the time.
 
A muffler with perforated tube can flow more air than the same size pipe because of reduced surface area , just some trivia.
 
PM999S said:
While there are loads of threads on pilot/mixture settings, my problem doesn't seem to fit with the many very good suggestions. I recently installed new exhausts on my '72 Combat using baffleless peashooters. Initially, I jetted up to 280 from the 230s that were standard to compensate for the reduced back pressure and went for a quick test ride. The bike certainly revved up quicker although I was still unsure whether the jetting big enough albeit the plugs have never been cleaner. Partial closing of the throttle didn't markedly affect the acceleration so I thought things might still be a bit lean.
This is where things started to go wrong, because I decided to try out the 300 jets and change the aging filter with an unused K&N for the standard airbox from the 1970s that I found on eBay to see if it improved breathing even further. However, while the acceleration remained strong, closing the throttle set off coughing and spluttering to the point of popping and backfiring. So, I went back to the original settings complete with the old filter but the problem remained.
I then looked for air leaks and saw the previous owner had tried to repair the carb rubber hoses with a bike tyre repair kit. Certain that this was the issue, I fitted new hoses after carb-clean spraying the pilot/mixture channels and checking the seating of the floats. No discernable difference. Speeding up the idle and richening the mixture screws have helped a bit but not massively. I've also checked the seating of the new headers, secured by new brass roses and gaskets, and there is no movement there to let air in. What have I missed?
Click to expand...
I regularly install Amal PACK 140 on Combats with no changes from the Amal setup except I use #19 pilot jets. Most likely the same peashooters as you have (LOUD), hamcan, close to sea level.

Model: Commando Combat
Right Carb: R932/19
Left Carb: L932/20
Main Jet: 220
Needle Jet: 106
Throttle Valve: 3
Pilot Jet: #19
Needle Position: 1
Needle: Std
Spray Bar: Std

So far, I've found no reason to change. I have tried the needle in the middle mostly to keep the engine a little cooler but haven't noticed any significant change.

So, I think you have WAY to big a main jet. If the back firing is a really loud bang, it is not actually back firing but rather explosions in the exhaust pipes cause by being over rich.
 
marshg246 said:
I regularly install Amal PACK 140 on Combats with no changes from the Amal setup except I use #19 pilot jets. Most likely the same peashooters as you have (LOUD), hamcan, close to sea level.

Model: Commando Combat
Right Carb: R932/19
Left Carb: L932/20
Main Jet: 220
Needle Jet: 106
Throttle Valve: 3
Pilot Jet: #19
Needle Position: 1
Needle: Std
Spray Bar: Std

So far, I've found no reason to change. I have tried the needle in the middle mostly to keep the engine a little cooler but haven't noticed any significant change.

So, I think you have WAY to big a main jet. If the back firing is a really loud bang, it is not actually back firing but rather explosions in the exhaust pipes cause by being over rich.
Click to expand...
Thanks for the reaction but the issue didn't manifest itself when the straight-throughs and 280 jets were first fitted - the bike breathed well enough on both open and closed throttle. It was only after I continued to muck about afterwards that things went wrong and I can't get it back to that first test ride...
 
PM999S said:
Thanks for the reaction but the issue didn't manifest itself when the straight-throughs and 280 jets were first fitted - the bike breathed well enough on both open and closed throttle. It was only after I continued to muck about afterwards that things went wrong and I can't get it back to that first test ride...
Click to expand...
Sometimes you just have to not worry about logic - too many variables. Make it stock, make sure the slides and needles are moving up and down properly, and give it a try.

There is no doubt, as far as I am concerned, that you are way rich on the main jet. But I can't tell from your writeup if that matters. The main jet is only really important from about 3/4" throttle to WOT. You also said that the air screw helped - that has no affect at more than about 1/8 throttle.

So, tell us the symptoms at idle, 1/4, 1/2, 3/4, and full throttle

If you've never seen/read this: https://amalcarb.co.uk/downloadfiles/amal/amal_tuning_guide.pdf
 
marshg246 said:
Sometimes you just have to not worry about logic - too many variables. Make it stock, make sure the slides and needles are moving up and down properly, and give it a try.

There is no doubt, as far as I am concerned, that you are way rich on the main jet. But I can't tell from your writeup if that matters. The main jet is only really important from about 3/4" throttle to WOT. You also said that the air screw helped - that has no affect at more than about 1/8 throttle.

So, tell us the symptoms at idle, 1/4, 1/2, 3/4, and full throttle

If you've never seen/read this: https://amalcarb.co.uk/downloadfiles/amal/amal_tuning_guide.pdf
Click to expand...
Thanks for the guide, Greg, and I'll take your advice on returning to stock after I've changed the manifold seals and perished balance pipe. The bike breathes perfectly from 1/4 throttle onwards so my focus is still on an air-leak and pilot/mixture screws.
 
Could carb sync or throttle balance have been compromised during your messing about? Check the tiny o-rings on the idle air and throttle stop screws. If there is a damaged one it will leak air and mess up idle control. What about choke slides if present? Could they be in closed position by accidentally putting control lever wrong direction or just stuck down?
 
Nothing to do with this problem other than terminology so that it's easy to follow. Backfiring is back through the intake while after firing is out the exhaust. It sounds like this may be fuel in the exhaust.

I've always thought of it as backfiring = air, looking for fuel; after firing = fuel, looking for air.
 
JimNH said:
Nothing to do with this problem other than terminology so that it's easy to follow. Backfiring is back through the intake while after firing is out the exhaust. It sounds like this may be fuel in the exhaust.

I've always thought of it as backfiring = air, looking for fuel; after firing = fuel, looking for air.
Click to expand...
It's definitely the latter, with popping through the exhausts not the air intake as far as I can tell on the run, and only on closed/feathered throttle as everything is smooth and strong on the open throttle
 
PM999S said:
It's definitely the latter, with popping through the exhausts not the air intake as far as I can tell on the run, and only on closed/feathered throttle as everything is smooth and strong on the open throttle
Click to expand...
So, dumping enough fuel in with a closed or slightly open throttle to get exhaust bangs! Not sure how we started on main jets! Sounds like a stuck/miss-adjusted float(s) or chokes on. Do you have chokes?
 
As a general rule the main jets come into play at 3/4 throttle they do have some influence at smaller throttle openings I believe but not much
 
marshg246 said:
So, dumping enough fuel in with a closed or slightly open throttle to get exhaust bangs! Not sure how we started on main jets! Sounds like a stuck/miss-adjusted float(s) or chokes on. Do you have chokes?
Click to expand...
The main jet was just the work that led to the issue starting so it was something that I did after the first fitting (that ran perfectly) and subsequent fittings. Chokes are fitted and work fine to the point they smooth out the issue by making the engine rev higher so I don't see them as the problem.
 
Tornado said:
Could carb sync or throttle balance have been compromised during your messing about? Check the tiny o-rings on the idle air and throttle stop screws. If there is a damaged one it will leak air and mess up idle control. What about choke slides if present? Could they be in closed position by accidentally putting control lever wrong direction or just stuck down?
Click to expand...
Slides and needles move perfectly in sync and while the o-rings on the idle/throttle screws look OK I have new ones on order just in case.
 
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Also check the tube on top connecting the manifolds, it likes to crack and leak or even pop off one side.
 
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