Honda-SOHC
General => New Member Introductions => Topic started by: weetchie on June 04, 2026, 01:25:35 PM
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Hi everyone i am a 62 year old biker and i am restoring a Honda cb400f 1976 i am having problems with low compression of 95 across all four cyclinders which i think is effecting the running of the bike as it will not idle off the choke i have rebuilt the carbs twice but when i try to sync them only one carb is pulling a vaccuum if there is anyone who has a good set of barrels they dont need and would sell or if anyone who lives near Didcot oxford that could help me get the bike on the road i would be very grateful thanks
mike
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Have you tried squirting some oil down the bores, to see if the compression rises to normal, just to rule out valves & head gasket?
What about the rebore option?
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Has it been sitting and not running for a while ?
If so, the following can help.
The rings could be gummed/stuck and not effectively sealing themselves well.
A non intervention (still assembled) method to encourage un-gumming them may help you. Remove plugs, spray into cylinders "Asda maintenance spray" (similar to many dispersal sprays) as its effective in acting on dissolving this type of deposit. Rotate engine with kick-start slowly, then leave it with ignition halfway between firing T marks, that should leave all piston halfway up their bores. Spray in more to each cylinder and leave overnight.
It should be on it's centre stand to fo this.
After soaking, crank with plugs out, refit plugs snd run engine to assess if there's any change. Let us know if there's any change at all.
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hi thanks for replies i have done a top end refresh with new rings honing the bore stripped top end removed the vavles and re lapped them new gaskets used reset tappets to 0.005 and timing is spot on but it still won't idle and when i try to sync carbs only number 2 is pulling a vacuum on my Carbtune pro i have been struggling for about 2 months i have ultrasonic cleaned carbs twice and bench synced but still no joy, i think its because of the low compression but not sure
thanks
mike
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Appreciate this is pretty basic and please don’t be offended, but are you absolutely certain that the valve adjustment has been done correctly. It’s not unusual to get this wrong and find that pressure is being lost due to partially open valves….just saying. It’s just that your statement about “not pulling a vacuum” is very telling.
Oh, and I’d be very inclined to double/ triple check the valve timing too, as a tooth out could also be an issue.
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Ah ha, different scenario then :)
As Sesman notes, double check camshaft timing to make sure there's no mistake there. They'll start snd run at a tooth out, but not correctly ------ and certainly done before, to much consternation. :)
Carb sync, bench sync is absolutely fine to both get running and avoid faults tying you in knots, don't go near it with any gauges while diagnosis is happening. It compensates for nothing in the engine and there's only one correct position for them, which decent method in bench sync will achieve.
See if you can get definitive answer on cam position first, then judge where to go.
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Welcome here Weetchie! ;D
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Before my 400 engine rebuild it took a lot of cranking to get it started, once it had run it would then start easily for the rest of the day. In my case it was poor cold compression.
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“….no vacuum…”. Says it all. Timing or valve clearance. But I strongly suspect clearance.
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Welcome to the forum.
Idle with a bit of choke on points to idle circuit clogged. If they are clogged it will affect other lowish throttle settings as well. If you hold up the idle jet to the light can you see a pin hole of light through it.
The cabtune screw in adapters are not the best for getting a good seal on the ports. Mine are plastic and always concerned with them breaking off if I nip them up to tight. As a side note, I have never improved on. Bench sync using this kit.
If it's producing some compression it must be getting air from somewhere either valve not closing when it should or sucking air in the rubber inlets, both would lean out the idle because it's not sucking a full dose of air/fuel through the carbs.
If you're getting the same compression across the board that points to something common to all cylinders, valve timing or clearances set with the piston in the wrong part of it's cycle.
If the valve timing is out it's not getting a full cylinder of fresh air/fuel to compress.
Regards
Dave
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NO VACUUM he says. So it ain’t getting air from the intakes. Or have I missed something? Does it actually have pistons😀😀.
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I would be inclined to connect the vac guages to a different port or use a few t pieces to connect them to the same port to verify that they read and respond the same. If the engine is running in some fashion somehow that doesn't correspond to no vacuum. Also if the throttle is not closed(throttle stop adjusted) as would be the case if your relying on pulling fuel through the main circuit with a bit of choke, the vacuum might be below the minimum reading of the guages.
Regards
Dave
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But is only running on one cylinder. I suspect the valve clearances….i’ve seen this before.🫢🫢🥲
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hi everyone thanks for all your suggestions i think i best start over and check from timing to valve clearances i dont think it the carbs as i have 2 sets and it's doing the same thing on both sets i will let you know when all is completed thanks again
mike
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👍🤞
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Welcome to the forum and you can see already there is plenty of advice. You have definitely come to the right place.
I'm with Sesman on this, check the valve clearance first, it really is easy to set them incorrectly and not realise because the clearance is so small and you think it is on compression and actually it isn't!
If that doesn't work take the rocker cover off (after first slackening the valve adjusters right off to avoid causing damage when you put it back on as you'll be trying to put the valves under compression as you do the mounting bolts up) and check the valve timing is correct using the sprocket marks. When you replace the rocker cover use a torque wrench on the bolts because they are really easy to strip.
Good luck and report back how you get on👍
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Hi Weetchie and welcome to the forum. Great suggestions on here to get you sorted, hopefully! Interesting to find out how you get on.
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Hi Weetchie
Welcome to the forum, the members of this forum are very knowledgeable and you will receive plenty of advice and wisdom.
Good luck with chasing down your fault
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Welcome to the forum.
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Any progress?
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Love to hear the outcome of this one…