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Post by scott7 on May 28, 2017 7:08:54 GMT -5
Hey guys i need help understanding low compression.i installed a 47mm bbk.
had 160lbs compression with a bigger head or more volume head but stock size valves.
so I decided to put a big valve performance head in which I checked first to make sure valves were sealed.
i lost a lot of power and have been trying to tune the the with bigger 21mm carb ,4.5 variator rollers and can't seem to get over 35mph when I was getting 37 to 40 mph.
So stupid me finally decided to do compression check on big valve head, only 90lb compression .
whats up with that .i don't understand ,my thinking was a bigger carb would allow more air to be compressed.
Is the bigger head only good for 50mm bbk ?
I guess I'm gonna go back to smaller head.
but I would like some feed back from you all so I can understand better.
Thanks
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Post by acvw74 on May 28, 2017 8:07:00 GMT -5
If your sure the head was good...I would expect the compression to drop a little with a bigger head...but not that much.
I would start my checking timing and then valve clearance.
Did the head gasket fit properly?
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Post by scott7 on May 28, 2017 19:45:30 GMT -5
Yea everything was good timing ,valves adjusted properly.
I went and put the other head back on and I'm back up to 38 to 40 mph
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Post by acvw74 on May 28, 2017 20:26:58 GMT -5
Did the head look ok when you took it off?
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Post by scott7 on May 29, 2017 17:15:58 GMT -5
Yea it looked fine ,I think it was just to much volume. But I put the smaller valve head back on and with the 21mm carb with a 91 jet.also had to put 1500 springs on the clutch cause made the back wheel turn even at idle. Then I keep lowering variator rollers all the way down to 4 grams. On take off rpms hit about 8200 and when I get to top speed I'm only at 7600 rpm. But I'm doing 40 to 42 mph. I'm very happy about that. Now I'm wondering if I should go down to 3.5 rollers. But I'm not sure what this stock crank can handle as far as rpms.
So I guess I'm wondering what is the max rpm I should avoid?
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Post by 90GTVert on May 30, 2017 8:43:42 GMT -5
There have been some complaints of low compression with some BB/BV heads even on the 50mm bores, so there's a good chance that the combustion chamber is too large.
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Post by scott7 on May 30, 2017 13:42:19 GMT -5
Thanks for imput, yea that's the only thing it could be, cause valves were sealed and so was head gasket.
So what is the top rpm I can do safely with stock crank ?
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Post by 90GTVert on May 30, 2017 14:04:34 GMT -5
With the larger bores, I'd think it's beneficial to keep them below 8000RPM if possible if you want it to last. It should be able to rev higher than it would need to or make good power at before any sort of failure occurs in very short order though.
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Post by scott7 on May 31, 2017 6:42:28 GMT -5
Thanks 90gtvert , when taking off it goes to around 8200 rpm maybe a little higher .then drops into the 7s and when I'm in top end its around 7600 to 7800 rpm with 4gram rollers.
I have 1500 springs in clutch so the tire won't move at idle and doesn't move till around 3000 or so rpm, I'll have to recheck that to make sure.
So is there any sense in putting a 1500 contra spring in. I'm running stock spring now. When I put 1500 contra in before to try , I couldn't open it to put belt in it . I was worried if I broke a belt on the road I wouldn't be able to replace it ,
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Post by 90GTVert on May 31, 2017 10:46:35 GMT -5
RPM sounds pretty good to me. If you aren't seeing black marks from the belt slipping or noticing other signs of belt wear and performance is good, then you shouldn't need to change the contra spring. I rarely use over a 1000RPM spring in any of mine because I haven't found it necessary and like you I don't see the need to make service more difficult just for giggles.
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Post by crawford on May 31, 2017 10:51:05 GMT -5
try a spoon full of oil in spark plug hole if rings are bone dry you may have low if it come up start it run it break it in and check compression again.
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