Author Topic: Revs wont go over 2500rpm  (Read 169 times)

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Offline CHOPM

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Revs wont go over 2500rpm
« on: December 06, 2018, 04:42:34 am »
Hi guys, first time asking anything but I'm rather stuck.
I have a 750 K4 and have it running but it just wont rev over 2500 rpm. I rebuilt the carbies but I'm still to tune them and vacuum test them. I've compression tested the cylinders and 1,2 and 4 are 110 psi but 3 is 75 psi. Could this be a problem or is this more of a misfire/ ignition timing problem or just running lean or rich? The bike was not started for 6-7 years before hand.
Thanks and any help is appreciated

Offline ekpent

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Re: Revs wont go over 2500rpm
« Reply #1 on: December 06, 2018, 04:59:40 am »
 One of the first things to check is that all 4 cylinders are firing. Easy test is to run for a little bit and then touch a wet rag to the exhaust head pipes and listen for a sizzle. That will tell a lot and get you pointed in the right direction. Other factors could be fuel flow,points and condensers condition,proper use of the choke lever,spark plugs etc. etc..

Offline calj737

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Re: Revs wont go over 2500rpm
« Reply #2 on: December 06, 2018, 05:10:22 am »
Does the throttle cable move far enough to fully open the carb slides? Is the throttle stop screw in the way? Did you bench synch the carbs? Are the cables bound up?

If you have had the carbs apart, there are many small mechanical things to verify first...
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Online Deltarider

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Re: Revs wont go over 2500rpm
« Reply #3 on: December 06, 2018, 06:41:49 am »
Simple tests first.
Try without airfilter element.
Kick several times and try to verify by either listening or watching, all 8 tappets move up and down.
If you have 4 exhausts let helper crank while you feel at the end of each exhaust with your hand if 'pufs' are there and more or less equal.
If it has sat for 6-7 years, you may want to verify the advancer is still moving.
Before diving into the carbs, make sure all aspects concerning the ignition are OK.
« Last Edit: December 06, 2018, 06:53:18 am by Deltarider »
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Offline flybox1

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Re: Revs wont go over 2500rpm
« Reply #4 on: December 06, 2018, 07:00:48 am »
Do all of the periodic maintenance items listed in the Honda Manual.
This alone, if done properly, will cure the majority of issues these bikes have....and it also gives us a better idea (since we know where your baseline is) of where to go....
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Offline CHOPM

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Re: Revs wont go over 2500rpm
« Reply #5 on: December 06, 2018, 07:46:12 am »
Thanks guys i will check more into it tmw. Cylinder 3 last time i checked the exhaust was really hot but the others were stone cold, does this mean they are not igniting? Could the engine run on just 1 cylinder? Ive checked the sparkplugs and all sparking. Throttle is opening the whole way aswell. I will have a good look tmw. Thanks

Online Deltarider

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Re: Revs wont go over 2500rpm
« Reply #6 on: December 06, 2018, 07:55:53 am »
Thanks guys i will check more into it tmw. Cylinder 3 last time i checked the exhaust was really hot but the others were stone cold, does this mean they are not igniting?
Could be, but not necessarily.
Quote
Could the engine run on just 1 cylinder?
Doubt it. Mine wouldn't.
Quote
Ive checked the sparkplugs and all sparking.
How did you test? Sparking when removed does not mean they'll spark under pressure. Is battery well charged? What do the plugnoses look like? When in doubt, renew. Plugs are cheap. Verify advancer functions as it should.
Verify fuel is in all 4 floatchambers. Any chance there's water in fuel? How old is the gas? Use fresh gas and/or startpilot (spraycan).
Quote
I rebuilt the carbies
I hope you didn't separate them from the rack. Are you sure slow- and mainjets are clean?

« Last Edit: December 06, 2018, 08:17:39 am by Deltarider »
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Offline CHOPM

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Re: Revs wont go over 2500rpm
« Reply #7 on: December 06, 2018, 05:39:37 pm »
So i have let the engine run for a bit this morning and used a wet rag on the exhausts, cylinder 2 and 3 are hot 1 and 4 cold. I did have sparks on those plugs but ill check again now. So this would mean either coild packs, ignition contacts(but i had the cover off and the 1-4 cylinder contact was sparking) or fuel.

Pilot and main jets are clear, and fuel is in the float bowls. Fuel is new.

Offline CHOPM

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Re: Revs wont go over 2500rpm
« Reply #8 on: December 06, 2018, 06:00:45 pm »
Just checked the coils amd resistance of each are 4.8 Ohm

Offline ekpent

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Re: Revs wont go over 2500rpm
« Reply #9 on: December 06, 2018, 07:20:22 pm »
 The points fire the cylinders in pairs with 1&4 being fired on one set and 2&3 being fired on the other so I would be cleaning/checking the gap etc on the 1/4 set for starters.

Offline calj737

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Re: Revs wont go over 2500rpm
« Reply #10 on: December 06, 2018, 09:53:39 pm »
I’d be checking the condensers. More common for them to fail than a coil. Swap them from 1/4 to 2/3 and see if the issue follows the condensers.
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Online Deltarider

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Re: Revs wont go over 2500rpm
« Reply #11 on: December 06, 2018, 11:07:55 pm »
So condenser (1+4), sparkplugs 1+4 and sparkplugcaps 1+4 are the first to suspect.
To check condensers, read: http://forums.sohc4.net/index.php/topic,171571.msg1996948.html#msg1996948 Bear in mind that they can show OK, but later, when warm, not!
Plugs must be good!
Plugcaps can be measured and should show around 5kΩ resistance. BTW, I do not recommend to run resistor plugs and resistor plugcaps combined. Check also HT leads/caps do not arc to the head.
« Last Edit: December 07, 2018, 01:06:06 am by Deltarider »
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