'73 CB350F air/fuel setting and proper size jets for straight pipes?

I bought a '73 cb350F with 7500 original miles on it / engine serial #1004889. The bike won't go above 70 mph. On a similar note after running at 60 mph, for less than two minutes, it begins to pop (fire coming out of the pipe) and it loses power. I have to drop to about 50 mph for it to run normal. Can anyone suggest what size jets and air/fuel mixture setting to use for straight pipes or if I'm even headed down the right path? The bike is pictured below to give you an idea about the pipes. I have #75 jets in it now and the mixture screw at 1 ½ turns out. Also, the throttle lags bad going back to idle when I let go of it. I bought it about 2 months and everything has run fine until I tried to take it above 50 mph.

Thanks in advance for the help.

Might not want to hear it, but I would recommend ditching the straight pipes and finding a nice 4-1 with a muffler, and retaining the stock jets. What is your air intake set up? Stock? If so, you're not gaining anything with straight pipes. A CB400F header will bolt right up, but you'd need to change the peg set up (CB400F pegs work great, mount right up, and they are 6" back from the stock 350F pegs). My 350F with that set up (stock carb/airbox, 400F exhaust) holds 75 easily and has some power left. More importantly, it runs like a top. I've had it set up like this for 15 years, no problems.

If you are dead set on running straight pipes with no back pressure, sure, you're going to have to rejet, think about your air intake, etc., as well as the longevity of your engine and the hours of headscratching, tuning all four carbs, retuning, etc.

I'm not dead set on the straight pipes, that just happens to be what she came with. I'm currently in the process of saving up to fix one piece at time. The next thing on my list are tires. I was keeping my fingers crossed about hoping for a simple solution that would keep me from having to drop a couple of hundred bucks in the meantime.

Have you checked timing and compression recently?

No I haven't checked the timing or the compression yet.

That might be a good place to start.

Timing and compression issues can often mask other problems and those two things are quick and easy to check if you have the correct tools.

Ok... So I had some fuel restriction problems which were easily fixed by completely cleaning out the carbs and the pet cock. There was some sediment clogging up both. Then I set the air/ fuel fuel mixed to the suggested 3/4 turns out and attempted to set the timing. As of right now I can't adjust the timing any more to get it to line up with the center line. I turned it as far as the adjustment holes will allow it to move. I got the timing a little closer to where it needed to be but its not even close. I set the points gap thinking that would help, but didn't. What would cause the timing not to adjust to center?

On a similar note, the adjustments made gave me 20 more mph and she's running much better than she did before.

The procedure for setting timing should be to set the gap first, then the rotation of the plates. Adjusting the gap will adjust the timing, which is why you gap the points first.

If you can't get the timing right (and you are 100% sure you're doing it correctly) then the problem is with your cam or your points. It may be that the points are too worn or that you cam was installed a tooth or two off of what it should be.

It's important for me to say thanks for the advice. So far is has been a huge help!

This might be a dumb question... is it possible for the cam chain to have been installed incorrectly at the factory, or is that unheard of?

Its pretty unheard of, as it would have been fixed when the first guy bought it under warranty unless he was a complete dipshit.

But on a 35 yr old bike its unlikely that no one has ever been in that motor.

Your main jet is probably a little low for open exhaust (which it will never run "right" with pipes that short) you could go up to an 80 and see what happens.

When you say it wont go over 70, how does it get there? As far as carbs its more of a throttle position thing, so how does it do WOT? Half? 1/4?

Anything's possible, but that would be one major fuck up and I've never heard of something like that happening before. Much more likely that someone has had the engine open before.

It only takes an hour to verify, if you're wanting to check it.

Take the stator cover off and turn the engine over until the LT mark lines up with the arrow on the stator. Then take off the rocker cover and you should be able to see some marks on the camshaft gear (to which the timing chain is attached) that are located in between the bolts that hold the gear to the camshaft. These marks should be parallel with the top of the head when the crank is at TDC.

Be careful putting the rocker cover back on. Only tighten things down after you verify that the rocker arms are not bound up with a valve stem. Also, every time the rocker cover comes off, valve clearances must be reset.

SONICJK said:

When you say it wont go over 70, how does it get there? As far as carbs its more of a throttle position thing, so how does it do WOT? Half? 1/4?

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Most of the Hondas use CV carbs, so engine RPMs is more important than throttle position when troubleshooting mixture problems, in most cases.

Sonreir said:

Most of the Hondas use CV carbs, so engine RPMs is more important than throttle position when troubleshooting mixture problems, in most cases.

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This is very true...
But the 350F is not a CV carbed bike so throttle position is more important it has linkage operated slides.

jtrout7 said:

I'm not dead set on the straight pipes, that just happens to be what she came with. I'm currently in the process of saving up to fix one piece at time. The next thing on my list are tires. I was keeping my fingers crossed about hoping for a simple solution that would keep me from having to drop a couple of hundred bucks in the meantime.

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Good prioritization. Tires are important.

As was mentioned earlier, it will never run right with short pipes and no mufflers and it will be annoyingly loud, so it's time to start looking for a pipe. Give teh bike a complete check up and service. Get a good manual and work through that to learn everything you can about it.