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Author Topic: high speed power loss?  (Read 2212 times)

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Offline orkid1989

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high speed power loss?
« on: October 01, 2011, 07:19:53 PM »
just finished rebuilding my 71 cb750 with the cyclex 850cc kit. has a 4 to 1 exhaust and pod filters. 130 main jets and 40 pilot jets and the needle raised a notch (clip dropped a notch). finally got it tuned in and it is running great. but i went riding on the highway today and decided i would see what it can do so i turned the throttle and got to about 80 and the power curve just started to disappear and i couldnt get past 90. any ideas what it might be? i read something about pod filters being effected by side winds so could that be it?
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Offline phil71

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Re: high speed power loss?
« Reply #1 on: October 01, 2011, 08:19:43 PM »
have you done a plug chop? Are you certain your petcock is flowing normally..
If it can't keep up with the demand of an 850 at WOT, you could basically be running out of gas as the bowls get low and aren't replenished fast enough.
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Offline dave500

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Re: high speed power loss?
« Reply #2 on: October 01, 2011, 09:55:39 PM »
your ignition is up to scratch?hasnt got a sticky advancer?
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Offline Danno

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Re: high speed power loss?
« Reply #3 on: October 01, 2011, 10:09:20 PM »
I agree with phil it sounds like fuel delivery problem not getting enough gas quick enough if that is the case your plug chop at high speed should look fine if it is white then I would think you maybe need 42 slows and your mix maybe set too lean

as far as fuel delivery problems go it could be the petcock not flowing enough or it could be the float bowl level is not high enough for high speed
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Offline dave500

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Re: high speed power loss?
« Reply #4 on: October 01, 2011, 10:16:12 PM »
try maintaining as high a speed as you can in fourth and see if it happens,,you can power through and high rev through all the gears cleanly?pods.
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Offline orkid1989

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Re: high speed power loss?
« Reply #5 on: October 02, 2011, 12:54:51 AM »
Just checked the jets and the 1 and 4 look ok but the 2 and 3 look like its running rich. Im so confused. Also timing is dead on and no sticky advancer. I have a brand new shut off valve and brand new 1/4 inch fuel line.
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Offline orkid1989

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Re: high speed power loss?
« Reply #6 on: October 02, 2011, 06:36:21 AM »
Their isn't a huge difference but its noticeable. Ill check the float levels again to be sure. Like I said it runs great. Just doesn't want to go fast
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Offline orkid1989

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Re: high speed power loss?
« Reply #7 on: October 03, 2011, 08:57:07 AM »
Took the bike for a test ride (a mile or so). Still had power loss. Then I yanked off the crappy pod filters and runs great. So I guess its just clogged up pods?
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Offline orkid1989

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Re: high speed power loss?
« Reply #8 on: October 03, 2011, 09:28:49 AM »
Do you think that could also cause the 2 cylinders to be rich and 2 be lean?
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Offline splitt

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Re: high speed power loss?
« Reply #9 on: October 03, 2011, 07:10:04 PM »
If two pods of the pods were plugged, it could cause a rich condition on those two cylinders.
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Offline orkid1989

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Re: high speed power loss?
« Reply #10 on: October 04, 2011, 10:00:59 PM »
just checked my compression and it was weird. 1 and 2 read 150psi but 3 reads just over 100psi and 4 is 90psi. put a small amount of oil in the cylinders and the compression went up so guess that means rings. my question is how can rings fail that fast? only have a few hours run time max and it never got extremely hot. im so confused by the bike at this point
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Offline nancy

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Re: high speed power loss?
« Reply #11 on: October 04, 2011, 10:55:11 PM »
Was this a rebuilt motor and NEW rings? Did u take it easy and run it in before hooting at 80mph? Incorrect ring fitment on the offending cylinders somehow?
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Offline orkid1989

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Re: high speed power loss?
« Reply #12 on: October 04, 2011, 11:01:41 PM »
Quote from: nancy on October 04, 2011, 10:55:11 PM
Was this a rebuilt motor and NEW rings? Did u take it easy and run it in before hooting at 80mph? Incorrect ring fitment on the offending cylinders somehow?
yep i used the cyclex git kit. what do you mean by incorrect fitment?
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Offline nancy

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Re: high speed power loss?
« Reply #13 on: October 05, 2011, 01:25:35 PM »
I agree with King's comments to wait.
In answer to your Q.....rings might be installed in many ways - but only one way is as per the book. Rings gapped badly, rings not overlapped, oil ring on upside down, rings not in spec with the bore wear. I have NEVER torn into this motor - so am poorly qualified to advise you further - but I speak from past experience on other engine misadventures!
Regards
Mark
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Offline orkid1989

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Re: high speed power loss?
« Reply #14 on: October 05, 2011, 10:53:46 PM »
Well it was getting a little of oil blow by. So I took apart the ring is stuck in the groove in the piston. So faulty piston maybe?
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Offline nancy

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Re: high speed power loss?
« Reply #15 on: October 06, 2011, 02:16:47 PM »
I guess it could be piston. If new rings are fitted to an orignal piston - my machinist would machine the ring grooves to accomodate rings. If u are installing new rings to new pistons - perhaps u have a cast fault in the ring groove? Mismatched ring sets? I guess you should see be now...what was the outcome here?
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Offline orkid1989

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Re: high speed power loss?
« Reply #16 on: October 06, 2011, 02:45:42 PM »
Quote from: nancy on October 06, 2011, 02:16:47 PM
I guess it could be piston. If new rings are fitted to an orignal piston - my machinist would machine the ring grooves to accomodate rings. If u are installing new rings to new pistons - perhaps u have a cast fault in the ring groove? Mismatched ring sets? I guess you should see be now...what was the outcome here?
new rings with new piston. Appears to possibly be a casting flaw in the piston. Called up cyclex and spoke with ken and he has been a huge help
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