Cylinder heads

paladin1176 · New Member Joined: Mar 04, 2007 Posts: 6

Well hello all. I am new to this site having just purchased 2 1974 tx 650A Yamahas. The first is in great shape and only has 9400 miles on it. The second has 22k miles and needs the head and carbs redone. I was told that there is a place that will rework the heads, plus harden the valves to use regular non-leaded gas. This is probably the first of MANY posts as I have never rebuilt a bike before. Nothing like having a project. I will try to post pics of them a bit later.

Thanks Very Happy

jayel · Posted: March 5, 2007, 3:53 pm Post subject: Re: Cylinder heads

at 22k if you pull it down for the heads don't forget to check/replace the cam chain/tension guide, if you are going to stay with a stock motor you can get it set up to burn reg gas but i would be tempted to hotrod it a little since you have another one as a stocker

paladin1176 · New Member Joined: Mar 04, 2007 Posts: 6

I wanted to rebuild this one as close to stock as possible. The running one has aftermarket exhaust, carbs, air boxes, front and rear suspension, handlebars, oil cooler and a few other pieces. It definetly rumbles as it rides. I have a few cycle shops here in Durham, NC that work on bikes, but nobody I want to give all my money to. I figured I would have to rebuild the motor.

royfisk · Posted: March 5, 2007, 4:32 pm Post subject: Re: Cylinder heads

The 74 is already unleaded from the factory and I have run the xs-1s and 2s on unleaded and never had a seat problem. The seats are what need to be re done on non leaded engines not the valves. Other then mayby some time porting it a little, unless its broke dont fix it. As for the engine if its all together do a compression test and see just where that engine really is. If it is low adjust the valves and do it again.If its still low then it probably needs top-end work. If the compression is normal clean the carbs , tune it up and start it. Now is there any weird noises? If not run it. If so its tear down time. I had one here that had sat outside and seized up and was brought to me 2 years stuck. I made up some witches brew and freed her up. tuned it up a little and started it. This bike today runs good and still has not been rebuilt with respectable 140 lbs compression.

paladin1176 · New Member Joined: Mar 04, 2007 Posts: 6

Here is what I was told when I bought the bike.
1) It runs on leaded gas and needs the lead substitute in it. Without it, the bike will run hot and not quite right.

2) The non-op bike has a burnt valve or 2 from having a lean run problem. Carbs should be rebuilt, plus the head needs to be pulled off, cleaned etc. Bike has only been sitting since 2002 and was running, although rough. It would need a little work to put it back together, but I could do a compression test with the right gauges. I take it 140 lbs is good, but what range am I looking for? I might get away with a crankcase full of marvelous mystery oil and a bit of work. Smile

jayel · Posted: March 5, 2007, 7:18 pm Post subject: Re: Cylinder heads

i own and ride a 74 tx unleaded no problem 93 octane it only pulls about 120 on compression, both cylinder should be within 10% of each other

-100 hard to start poor running, low 110, good 120/140, better 150, above 150 start brewing your own fuel mix nitro man

paladin1176 · New Member Joined: Mar 04, 2007 Posts: 6

If this guy has been running lead and 87 in it, is it going to play hell when I put 93 and no lead?

jayel · Posted: March 5, 2007, 9:05 pm Post subject: Re: Cylinder heads

nyah shouldn't be a problem, probably just run better just do a couple of high speed runs and check plug color (light brown to tan) with new plugs NGK B-8ES OR BP-7ES (lower #=hotter plug) on the NGKs, see what it likes with your riding style I run mostly highway 15 /25 miles@65/70mph between towns at a time on my 74 TX 2 to 3 hundred total in a day running the B-8s seems to work fine

grizld1 · Posted: March 6, 2007, 12:58 am Post subject: Re: Cylinder heads

If the scoot shows poor compression at first but fires and runs, give it time--rings sometimes take a couple hundred miles or so to free up and seal after a long time down, and compression may improve a lot. Also, when you do the compression check, remember that the slides in these carbs are opened by engine vacuum; so while you're turning 'er over for the compression check, do it with the slides either propped up or removed and the throttle open, so the intake tract is wide open.

royfisk · Posted: March 6, 2007, 8:14 am Post subject: Re: Cylinder heads

good point grizld; My would guess is that paladin has either a ignition timing and or cam timing issue with the symptoms describbed. Probably cam timing and it would be retarded a tooth off causeing the intake valve to stay open. Then to add to the problem someone previously diagnosed it as ignition timing and retimed the ignition and played with it to get the bike to run on high octane. And passing the bike off. Or the compression is extremely high and is a race only bike.

grizld1 · Posted: March 6, 2007, 9:27 am Post subject: Re: Cylinder heads

Good diagnosis, Roy. Got to agree--"Burnt valves" are probably not the problem. I've post-mortemed around a dozen dead XS650 engines over the years--holed pistons are common, but none of the salvage motors I've broken down had valves with enough wear or damage to stop the machine. That's not to say they were all in great shape, made of impervium, etc., just that an over-lean mix, maybe combined with a touch of iggie error to the advanced side, tends to hole pistons in these engines before the valves take terminal damage.

paladin1176 · New Member Joined: Mar 04, 2007 Posts: 6

Ok, so check the compression first. See if it's reading in the 120-150 range. If it's good there, clean it and drive. Also check for timing. I would imagine it's a bit more involved to time a motorcycle vs a dodge 318, and takes different tools.

grizld1 · Posted: March 6, 2007, 11:05 pm Post subject: Re: Cylinder heads

If it has anything over 100 lbs. I'd try to get it running and see if compression rises with use, babying the engine with some break-in miles while the rings free up and work in. Tension the cam chain and adjust the valves as well as the ignition timing. Tools for the ignition are the same--screwdriver, feeler gauges, strobe light. Be sure to confirm the advance.

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