D15B4 Misfiring unless under load or idle, happened after hitting 6k RPM

[90civicGL]

Okay so I let someone else drive the car and they did a pull to 6-6.2k in 3rd or 4th gear and after that we noticed that the car now misfires, first noticed it taking off at the lights. The car has been to 5.5k a couple times and no problems.

The car sounds and feels to be running fine while idling and under load, so pulling up hills above 3k rpm it's fine. Its most notable around the 1k-3k range. But will do it at any rpm if the accelerator is down a bit but not pulling. The engine is D15B4 Dual Carb.
Have taken the dizzy cap off and checked it. Pulled each spark plug out and checked it seem to all be okay.

Have a feeling it could be a clogged carby jet? Idles jets seem to be okay. But if this was the case wouldn't it also be a problem while pulling?

I don't think it is ignition, even though it's probably the most likely cause. I'm going to get out the timing light and compression tester tomorrow.

Anyone have suggestion or know if this is a known problem with the engine/hondas? Read that some of the newer hondas get hairline cracks in the coil packs which seems to cause a similar problem. Guess that could be a possibility? All help and suggestions will be greatly appreciated as I want to get it back on that road as quickly as possible.

Many thanks.
Nic

[90civicGL]

So did a compression test, left to right it was 200psi, 190, 180, 190. So third cylinder is the only real concern there, put a teaspoon of oil and redid the 3rd and it went up to 200. SO makes me think it's seal not valves. So I think that is just from an engine that has 415k kilometres on it. Ruling out the misfire being mechanical. Going to try timing light next.

[dc2r-0636]

Engine is still fine for its age. Sounds like a clogged carby so give it a clean.

If that fails I'd be leaning toward the ignition side of things.. I'd change plugs and leads first and then get a
Aftermarket distributor

[90civicGL]

Engine is still fine for its age. Sounds like a clogged carby so give it a clean.

If that fails I'd be leaning toward the ignition side of things.. I'd change plugs and leads first and then get a
Aftermarket distributor

That's my best guess at the moment as well. Should probably also mention the ignition leads and spark plugs only have 1500-2k kilometres on them. I'll clean out the bowl and jets and do the timing light check and report back.

[90civicGL]

Resolved!! Working on an engine and fixing a problem is the best feeling in the world I swear!!

Okay to curious readers and future people with this problem.

So carrying on from before I hooked up the timing light and spark was fine on all four leads, sparks were zapping the valve cover nuts all in time. Lets just say there was a lot of spark! More then I would have thought. So the next test we did was pull each spark lead out of the cylinder one by one. With the theory behind this that if the cylinder is causing a problem then when we pull out the one at fault there would be no/minimal change in engine behaviour compared to the other ones. What we found was that cylinder 1 and 2 counting from the timing belt both made the engine change noticeably. So these were the two good cylinders basically keeping the car running. Were as cylinder 3 and 4 didn't make as much of a change, engine just carried on like it has been. So these two are the cause to the misfire. Since it it was tied together with cylinders 3 and 4 linked and 1 and 2 linked this helped confirm the carbs were the issue. So we hooked up a vacuum pressure gauge the carb fuelling cyl 3 and 4 and it didn't show much vacuum but under hard throttle the needle moved a little. Did the same on the other carb which is in theory running better and the needle moved even less. Thought that was a bit odd.. But we were sure it was carb and not ignition at this point. So we wanted to remove the top cap off the carbs which lets us remove the spring and the needle valve for the main fuel jet so we could blow compressed air down there in hopes of unblocking it. As removing the bottom container and float would have been a PAIN! (Removal of intake system out of car) After pulling the top caps off we found the problem!! Yay!

What had happened is because the carbs had been sitting a while there was vanish and gunk build up where the piston looking needle valve travels up and down from the spring and vacuum. So my dad and myself normally just got up for 5kish sometimes 5.5k revs, and when my uncle drove it taking it to 6k/6.2k revs it pushed the needle valve up further due to the higher pressure jamming it with the gunk. So these weren't moving not allowing the carb to adjust the fuel. Checked the other carb and exact same issue. The engine ran very smooth before the 6k pull so the carbs were working before hand. After cleaning it al with carb cleaner it now slides up and down as smooth as butter. And the whole misfire problem has been resolved. And as smooth as it was before it has to be smoother now that all the gunk is gone from the carbies.

dc2r thanks for the help, was indeed a blocked carby, just not the main jet which I initially thought it was. Luckily my uncle felt guilty and had a fair knowledge of engines and carbs, so didn't have to spend a cent.

[dc2r-0636]

That's terrific news mate. Glad it was a inexpensive fix

[markismaximus]

was this the motor you bought from me? or not yet swapped it into your car? if so it only had 315K on it, not 415k. good to hear you fixed the issue though. The car was very well kept, so one would assume the engine was too. those are awesome compression results for a 23yo engine

[90civicGL]

was this the motor you bought from me? or not yet swapped it into your car? if so it only had 315K on it, not 415k. good to hear you fixed the issue though. The car was very well kept, so one would assume the engine was too. those are awesome compression results for a 23yo engine

I'm using the head and bottom end off my original one, and the carbs and intake mani off the one I purchased from you, and it has been working great. Had done 2500k's on it before this problem. I'd say the previous driver had never pushed the engine past the 5k mark which was why we didn't get the problem earlier. So taking it up to an rpm it hasn't been to in many year would have sucked the needle valve up to where there was gunk build up. Cause it was working flawlessly before, and works just as well now. Yeah, although with compression anything more then 10 between cylinders isn't ideal correct, or is it 10% making it acceptable? So my 180 might be a bit of a worry?

[markismaximus]

factory service manual states compression nominally is 185psi, minimum 135psi, maximum deviation between cylinders 28psi. the motor is healthy as for as compression goes

[tiksie]

B4's are awesome little motors

[90civicGL]

Okay, thanks for the figures mark, and yeah the D15B4 is an awesome little motor. The amount of engineering they put into it is great, the ED civic in general actually for an early nineties family hatch. Makes for a good car to learn to drive in.

[dove grey 64]

My wagon just developed the same issues after giving it a thrashing. Guess the previous owner never gave it a spirited drive, does currently have 475000ks unopened
Will report back when I have a look.
Thanks for the heads up