Loss of power 1979 CB750 - full throttle

[wassupitaly]

Tools, how do you check if the gas cap air vent is plugged or not?

[Rodgil]

Pull the fuel line off the carby to check the flow from the tank. That is the easiest way to check for blocked vent in fuel cap, or in-tank filter. And if you have a vacuum operated petcock, get rid of it. NGK have on online catalogue to get part numbers for any plug for any vehicle.

[Piute]

sli750f wrote

one more thang what would be the worst thang it could be?

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[wassupitaly]

Here is what my spark plugs looked like.
From left to right
1) white in color
2)felt wet
3)was and old one that was previously in there blackish but not wet
4) looks brand new.

1,2,4 were brand new. 3 was older one on the bike....long story

[wassupitaly]

So took gas tank off. Petcock valve to off. When I go to ON barely any gas trickles out. When I go to reserve nice and free flowing. I changed the petcock valve with this one -> http://classiccycledepot.com/index.php?productID=145

because I broke the nipple on the older one. Why would it be free flowing on reserve. There is gas in the tank. Haven't driven it that much since the last fill up (premium gas). I'm confused. I guess I'll try it on reserve the whole time....

[wassupitaly]

So my 4th spark plug still looks new because maybe it's not firing. First 3 exhaust pipes are hot, 4 one is hot but spit does not evaporate instantaneously like the other ones.

[robs750]

As far as the petcock,gas should be free flowing in "on" or"res".Sounds like something is wrong with the "on" posistion.

You need to check for spark on that one odd cylinder and check for fuel in the carb bowl.

One thing I learned with these old bikes is to check the valve adjustment,check compression and do a leakdown test if needed before moving onto other things.

I found a clean '82 cb750k for my son.The bike had 33000miles on it.Bought from the original owner.The bike was cold blooded but ran ok when warmed up,My son road for about 3000 mile before it had idling and starting problems.We assumed it was the carbs so I rebuilt them.The engine would not start after that.
So we checked the valve clearance and found that the exhaust valves were tight.Four of them would not even except a feeler gauge.So then a compression test was done and the test showed 40,60,40 in 3 cylinders and 120lbs,in one.So then we did a leakdown test.The exhaust valves are leaking and some of the intakes.

[wassupitaly]

I had fuel in carb bowl. there was a little dirt in the fuel when I drained all the carb bowls so that makes me think that the carbs jets might be dirty as well. Not sure when is the last time they were cleaned.

As far as the valve clearance and compression testing, I need to do those maybe this week so I will keep you guys posted on how things go.

What is a leakdown test?

[robs750]

A leakdown test is placing air into a cylinder to test the sealing of the valves,rings and headgasket.There are special gauges/regulator you could buy that show the percentage but are pricey.There is a flexible adaptor that screws into the spark plug thread.
With the piston at TDC and valves closed,you apply air pressure to the cylinder.If the valves are leaking, you will be able to hear air come out of the intake or exhaust or both.If the rings are bad,the air will escape into the crankcase.A bad head gasket will also show a air leak.
You can also do the test with an air compresser with a regulator to get a ball park check.

[alehman]

About your engine missing at high speed--- have you re-jetted your carbs at all or are you sure your jets are stock? The manual says that if the engines misses at high speed and it's not your ignition or cleanliness of carbs, the jets are probably too large.

[wassupitaly]

Haven't had time to open it up. Bought it couple month ago and not sure of the size of the jets.