Topic: CB400F misfire at idle

[Alaxy Galaxy]

So, riding season is around the corner and I've started to prep my CB400F.

I'm having an issue where (I think) my #1 cylinder isn't always firing at idle. I feel like I've been chasing this problem since I built the bike.

The bike doesn't idle very well and dies off after a few seconds. It works well as soon as you give it some gas, though. My diagnosed the problem by putting water on each pipe and noticing that it wouldn't boil on pipe #1 (although it does get quite warm). I then pulled the caps on each plug when the bike was trying to idle. All but the #1 killed the idle immediately, but pulling the #1 cap did nothing.

Last year I set the points and did the valves twice to no avail. I put a rebuilt rack of carbs over the winter, but I kept the brass from my old carbs.

My plugs from last year were all fouled and the #1 plug shorted, so last night I gapped and installed new plugs (D8EA) and it seemed worse. I swapped the #1 and #4 plug caps (they're original) and the problem stayed with cylinder #1.

I have a few questions:

-Could it be a syncing issue? I only bench synced these new carbs with an 1/8 inch bit. However, last year, with different carbs, I gauge-synced them and still had idling issues.

-Could it be my pilot jet? I cleaned every hole with guitar strings over the winter, but maybe it got clogged again?

-#1 plug hole has a helicoil in it. Could this cause enough resistance between the aluminium of the head and the helicoil metal that at idle it mostly sparks on #4 because it has less resistance?

-The compression on the motor is less than stellar (110-110-135-135, not sure which order). Could this be a factor at idle?

I'm at my wit's end. I honestly wouldn't mind paying a mechanic 400$ at this point to get it running right. 

[lafindufondu]

Probably clogged pilot. 

With those compression numbers I would think you need to rebuild that motor.

[Deltarider]

Probably clogged pilot. 

With those compression numbers I would think you need to rebuild that motor.

Caution! Not many can do a compression test on our CBFours succesfully...

[bryanj]

Its the variation thats the bigger problem rather than actual numbers

[Alaxy Galaxy]

Probably clogged pilot. 

With those compression numbers I would think you need to rebuild that motor.

Caution! Not many can do a compression test on our CBFours succesfully...

I did what most people do and used a push-in rotary gauge and cranked at least 7 pumps with the starter. What would you suggest?

[Alaxy Galaxy]

Could the mis-matched compression cause my idling issues?

[lafindufondu]

Not too sure, but you want all cylinders compression to be within 10%.

[Alaxy Galaxy]

I'm going to get the ignition issue figured out tonight or tomorrow. I made an inline spark tester with a neon bulb to see if there's good contact with the helicoil.

If the spark is good, then it's gotta be fuel delivery or something to do with the motor itself.

[BenelliSEI]

The pilot jet sounds suspecton #1. Where is the idle moisture screw set on that carb compared to the rest and definitely do a vacuum carb sync before you head off in any other direction......

[Alaxy Galaxy]

The pilot jet sounds suspecton #1. Where is the idle moisture screw set on that carb compared to the rest and definitely do a vacuum carb sync before you head off in any other direction......

All idle set screws are 2-1/2 turns out (from the Honda owner's manual). I checked with my home-made inline spark meter and all is ok (it looks really neat though!) when the bike is cold.

I will clean out the idle jet in #1 and figure out a way to sync them next. I tried making the tube and water thing last summer, but it was pretty much a disaster! 

Also, cold, #1 was at 90psi and #4 was at 110psi. 

[HondaMan]

Could the mis-matched compression cause my idling issues?

In a word, yes.
It is possible that one of the valves of the #1 cylinder are dirty. This will cause low compression, but it can sometimes be made to 'go away' or 'heal' by an old trick I used to use on customer's bikes (poor college kids, mostly...). This is: set the clearance of the valves on the low-compression cylinders larger, like 0.005" intakes and 0.008" exhaust. This will increase the temperature of the affected cylinders when running the engine steadily, and often 'cooks off' the carbon on the valves. It takes about 200-400 miles to accomplish.

If this doesn't work, then you are faced with having to pull the head to find out what the bug is.

[Alaxy Galaxy]

Could the mis-matched compression cause my idling issues?

In a word, yes.
It is possible that one of the valves of the #1 cylinder are dirty. This will cause low compression, but it can sometimes be made to 'go away' or 'heal' by an old trick I used to use on customer's bikes (poor college kids, mostly...). This is: set the clearance of the valves on the low-compression cylinders larger, like 0.005" intakes and 0.008" exhaust. This will increase the temperature of the affected cylinders when running the engine steadily, and often 'cooks off' the carbon on the valves. It takes about 200-400 miles to accomplish.

Huh. That's neat. Are you sure about the clearances? Stock tappet clearance on the CB400F is .0019" both in and out. The CB750 is .002" and .003", right? Should I just do .005" for both?

Also, here is my jury-rigged in-line spark checker. Please note that I shorted the resister inside the neon bulb (hence the need for heat-shrink to hide the hole).

[lafindufondu]

You can ground a spare spark plug to the engine to check for spark.

[Alaxy Galaxy]

You can ground a spare spark plug to the engine to check for spark.

Wouldn't that just tell me that the spare plug sparks? I wanted an inline spark tester for the plug in the head, as I was worried the helicoil was preventing a good ground contact. This doo-hickey worked great.

[bryanj]

Helicoil will not prevent ground contact as it is metal between metals

[Alaxy Galaxy]

Helicoil will not prevent ground contact as it is metal between metals

I guess I'm just being paranoid since I bought the engine with the helicoil in place. And after reading somewhere on the Interned that someone put epoxy on a helicoil to keep it in place... 

In any event, I'm just trying to eliminate all the possible causes one by one. The bike rides quite well and pulls really hard, it just doesn't want to idle. I guess it's a race bike! lol

[Alaxy Galaxy]

Whelp, new pilot jets fixed my problem.

My plugs were always fouled and after searching the forum (as one does), I found a topic about another member with similar problems when ended up putting in smaller slow jets.

This past winter, I swapped my carbs for a set of CB350F carbs because the P.O. had machined the carb throats for more airflow. I kept my original main jets, needles and slow jets, since the 350/4 jets are smaller. Everything else is stock on the bike, except for a pattern muffler. The 400 uses Keihin #40 slow jets and the 350 uses #35 jets. I put the 35s in and the motor is much better behaved and the plugs are no longer fouled. It runs really great and after a carb sync, it's so smooth.

Now as for those #40 jets... I can run a 17 gauge guitar wire in there, which means they're at least 43s so who knows what happened. 

[bryanj]

It is amazing how often you find that previous owners have drilled out jets instead of getting correct size and/or cleaning with abrasive rods!