Still doesn't feel right.

I rebuilt the top end of my 82 Seca. New rings, hone, lap valves, checked, tested, and everything to book spec.

I took it out yesterday, and it still didn't feel right. I have not had a bike like this in over 20 years, so it could just be me. I think it is still sucking air (new throttle shaft seals on the way) because it was running fine until I went to put the final adj. on the carb sync. As soon as I pushed on the screwdriver to adjust it, it reved up, and when I released pressure, it would idle down. It does idle down now, but not as quickly as I would expect it to.

I noticed that when I tore down the engine, the cams were off a tooth on from the crank. I set them to book spec (crank on "t" mark, and cams with indicator hole straight up)

I did a compression test before starting the engine, and compression is low - 90-100psi. The rings are on as stated in the book - gap positioned properly, ring markings up, all rings where they belong.

It runs ok, but there is a hesitation upon acceleration, kind of like a turbo lag, and it feels like it is lacking power. (from what I can remember of this bike) The old bike I had (83 Maxim) you would snap the throttle, and it would launch. I could peg the speedo in second gear. This bike seems to be geared differently. At 60 miles per hour it is running at 4k rpms.

Does this seem right to all of you with a 750 Seca?

Just wondering.

 

90 to 100 pounds compression is too low.

i have slight blowby but i have 140 pounds.

 

You sure about the cam timing thing? Take another look...

 

compression seemed real low to me to. The only thing I can figure is that the rings need to seat. It had better compression before I did the rering.

I checked the cam several times. The book says to bring the cams so the indicator hole lines up with the arrow on the clamp. Then bring the crank to the "T" mark on the timing plate. I did as the book instructed, and rolled over the engine by hand in both directions several times to be sure that it was correct.

The issue here is that the marks I put on the cam before disassembly do not line up. They now line up with the timing plate half way between the "C" and "T" mark.

I have rebuild several engines in the past, and have never had issues like this before.

 

We ran onto this recently on the CRF's forum after a CRF 250R rebuild. A couple of squirts of 10w30 in each cylinder will seal the rings to the cylinder walls and boost your compression back. Once the engine starts and runs (and smokes for a while) it should settle in. Rings are strange animals sometimes.

 

Getting the rings seated is VERY important and will make a huge difference.

Here is a good write up explaining what is happening.
http://www.ntnoa.org/enginebreakin.htm

Depending on how you run it, how good of a hone job, and how often you need to adjust the carbs it may not "settle down" until you get over 1K miles on the rebuild.

I needed to adjust the sync on mine about 4 times as the compression kept changing and valves settled in. She's pretty stable now after 1700 miles.
My compression is between 154-160 now (the X engine is a tad higher than yours)

 

compression is very low. the speedo cluster must be off they turn right at 4500 at 60 there is no way to change the gearing if it has a tall tire mike make 100-200 rpm change but thats about it compression should be over 140 designer mikes compression is good but it should be a bit higher then yours higher compression ratio. did you give it a good hone job? sometimes it takes a few hundred miles to get the rings seated properly hope its something as easy as that i dont think its a cracked head the cyls are all to close to the same compression did you recheck the valve clearence should be fine but you never know. also when your doing your compression test are you holding the throttle wide open

 

I did give it a quicky hone job. Just enough to get the glazing off. I didn't want to take too much off the cylinders. I lapped the valves and leak checked them - They are good. All valve clearances are good. I didn't have the carbs on when I did the compression test.

Today I popped off the cover and did a visual check on the cams. Using a screwdriver to get top dead center, the cams were symmetrical. I am assuming that the PO jumped a tooth on the crank, or assembled it wrong in the past.

It runs ok, just not as good as I would expect. When I pulled the plugs, they were a nice golden tan color, almost yellow. I will run it for a while and recheck everything.

Thanks for all the input.