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my 79 400 T has popping ..air pops it sounds like , at low rpms. at times you can see carbs move a tiny bit when this happens. Also after going it a bit of throttle , small back fires out the exhaust
 

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Didn't you have a similar question early this year?

Which carbs are you using, did you switch to the 450 carbs that were mentioned?
If you kept your 400 carbs did you put the stock jets, needle and seat, float and float needle back in?

First you'd want to be sure there are no leaks between the carbs and head. There are a few ways, most involve spraying starting fluid or carb cleaner around the boots. The clamps stretch with age, perhaps a strip of beer can between clamps and boots can tighten them? There is an o-ring between the boots and head that can be replaced easily.

When I got my CM400C running years ago it had a similar symptom, it was a slide needle problem, aftermarket needles have a different profile, stockers helped to cure it.

You should go through the CM400 ignition troubleshooting thread to make sure your ignition system is all good, these are the parts that keep your timing set, if something is bad your timing can go off.

You should set cam chain tension and valve clearances. I retorqued my head gasket before I did those things, some say not to though.

Then I set my idle speed and synced the carbs. After that it could rev to redline without any popping anytime.
 

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Discussion Starter #3
Didn't you have a similar question early this year?

Which carbs are you using, did you switch to the 450 carbs that were mentioned?
If you kept your 400 carbs did you put the stock jets, needle and seat, float and float needle back in?

First you'd want to be sure there are no leaks between the carbs and head. There are a few ways, most involve spraying starting fluid or carb cleaner around the boots. The clamps stretch with age, perhaps a strip of beer can between clamps and boots can tighten them? There is an o-ring between the boots and head that can be replaced easily.

When I got my CM400C running years ago it had a similar symptom, it was a slide needle problem, aftermarket needles have a different profile, stockers helped to cure it.

You should go through the CM400 ignition troubleshooting thread to make sure your ignition system is all good, these are the parts that keep your timing set, if something is bad your timing can go off.

You should set cam chain tension and valve clearances. I retorqued my head gasket before I did those things, some say not to though.

Then I set my idle speed and synced the carbs. After that it could rev to redline without any popping anytime.
i used the stock carb v 22 that came with the bike. sent the carbs to Brennys Motorcycle clinic in Iowa and they went through and cleaned and replaced anything was bad with new gaskets..they used most of the stock parts..clipped off a small part of the plug wires and checked the plugs..kinda gray tips...did spray and rpms sure increase..tks so now i know where to start...i did replace them last winter while putting everything together..ran great for awhile and now this..always something with the old bikes..started on a 78 suzuki gs 550...needs pants fro were,,did have it running thou....but not good
 

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You're describing lean pop thru the intake. Typical cause is the carb insulators. Next would be the carbs not fully inserted in the insulators, they bottom out on each other. Air cut valve diaphragms leaking, insulator O-rings failing/hard. Idle circuit not clean enough.
 

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Also check you valve clearance. tight intake valves can cause sputter through the carbs.
 

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On bikes with two separate, (single spark plug wire) coils...YES.....
On bikes that share one single coil ( but it has two spark plug wires).....NO
 

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Discussion Starter #11
You're describing lean pop thru the intake. Typical cause is the carb insulators. Next would be the carbs not fully inserted in the insulators, they bottom out on each other. Air cut valve diaphragms leaking, insulator O-rings failing/hard. Idle circuit not clean enough.
did the resistance test on coil , plug wires all tested a ok in the range but the plug caps tested under.5 about.42..both sides so ordered couple
 

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You'll want to measure your stator and pickup coils as well, having these in spec is very important for these bikes.
 

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Adjusted valve and turned screws in till idle drop to 1000 on both sides ..reset carbs on cylinder boots..still not as much now small baccarat and have noticed a flame popping carbs back
 

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Stator..pink , green wires were 101.shluld be 129...brown. light blue wez 132..should be 203..pick up coil..blue , white checked out ok.
 

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Your primary pickup and advancer pulsar are way out of spec which will cause the timing to not advance the way it should = loss of power. You need a good stator from a CB/CM 400/450 manual trans engine. There was one on eBay yesterday for $225, might be gone now.
 
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