Can't get a 1978 Yamaha xs750 running

[Toolmaan]

I bought a project. A 1978 Yamaha xs750 triple. Sat for 2 years because the fuse box went bad. I replaced the fuse box w/ inline fuse holders. Changed oil, replaced plugs, Tried to start. Tried to start but just popped back through the carb. I pulled carbs off and they were full of nast gas and dirt. I pulled them completely apart, blew through all jets, replaced the vac slide diaphrams, Replaced the intake boots because of cracks, put them back onthe bike, then I got it running but it wouldn't go over 2000 rpm. I put fuel filters on the fuel hoses out of gas tank, I capped off the vac hoses to the petcocks and set the petcocks to prime. Still same problem. I moved timing plate around, runs better with it turned all the way to the right, I let it sit a few days and now it won't start, I checked spark, and its weak. Not a blue spark kinda white, and it won't jump a 1/4" gap. All the 3 cylinders are the same way. I would doubt I have 3 bad coils. I've heard of guys having problemw with the CDI black boxes, any other ideas? I did buy a Used CDI box off of ebay but I don't have it yet. Thanks for any help.

[TheDjost]

Make sure your battery has a REALLY good charge. You can even boost the bike with you car. The battery may have gone bad from sitting too long. If you want to test it, make sur it's a load test.

If that doesn't work, I'd go through the carbs again. It's not like you have 4 of them like we do That 2Krpm thing is probably carb related. Just clean the hell out of it again. You didn't say if you changed the jets. If you did, double check the hole sizes, they may be stamped the same number but the holes might not be the same if they are aftermarket.

Hope this helps

-Justin

[xs1100yam]

Hi

To locate the problem (Carbs or ignition) you may use a spray (in France, we have "Start Pilote", you probably have equivalent...)

Disconnect tank and get the 3 carbs bowls empty, then remove air filter and spray in the air filter box.
start the engine.
If it run better (almost normaly) the problem is gas feed or carbs.
if it doesnt, problem is ignition

Patrick

[Bill__Woods]

hey toolman,
My friends XS750 would nto rev properly because the Governor (mechanical advance unit) was on working. We removed it and cleaned & lubes and that fixed the revving problem. His bike would rev to about 2700 rpm then fall on its face. and the symptoms were close to fuel starvation. Remove the left hand round engine cover and the governor is there. Look at the michrofiches at Bob Ryan's XS750/850 Triple site if you want to see the peice i am talking about.
Clean your carbs again for good measure, remove ALL jets , clean and blow compressed air thru all passages. Recheck float levels.
The spark on these bikes will not look as bright and hot as your new car. If you have spark on all 3 plugs, I beleieve your TCI box and the pickups are working correctly.
You need to check your service manual and make certian the the Timing Chain is lined up correctly. If it is , out your ignition pickups in the factory position.
Replace your 3 spark plug caps and the wires if you have not already. 3 NGK caps and some 7mm wire is less than $10.00 at any bike shop.
I agree with patrick, mist the inlets of the carbs with Carb Cleaner and see if it starts. If it does you have a fuel problem.
A friend of mine has another set of carbs and several parts from a 78xs750se, i can give you his email if you wish. I have 2 XS750 engines that I know run , out in the shed. I have a 18K engine (complete) and a 33K engine (long block -stator, stator rotor, starter, ignition pickups, stator cover, Ignition side cover). Need an extra engine? I am ready to deal on them since I know longer have my xs750.,
post back with your further test results and keep us updated on the progress.
Triples are cool.

[Toolmaan]

I've had that governor piece off, it all looks good, the advance moves freely. I have misted carb cleaner in the carbs and it hasn't made a difference. I did pull the valve cover and checked the timing marks but I couldn't tell where the cams line up. I have a Clymer manual but it doesn't help much on where to line up the cams. When I moved that governor piece to the right it would run better but still wouldn't rev very high. Right now it won't start at all, it has spark but like I sauid it seems weak. It doesn't crank over weird or anything like a cam was out of time, Compression is about 130psi on each hole. I have ordered a good used CDI ignition but I don't have it yet, I'm gonna just try it and hope that is what it is. I did replace the spark plugs, what do you mean by plug caps?

[xs1100yam]

Hi

I read again the posts, and it seems to me that the symptoms are "Ignition Out of Time", too much advance...

I don't think you're "out of time" with camshaft and valves. (I hope for you).
Only 1 tooth error between chain and camshaft sproket may produce severe damages:
- bended or broken valve tail
- piston head
- cylinder head
In this case, there is no compression at all and it is faster and cheaper to buy a second hand engine

Anyway, just to be sure, it's easy to check:

The dots on the cams must be aligned with the arrows on N° 1 cam caps. (the caps are numbered from left to right: 1, 2, 3, 4. The arrows on the caps point to the left)
The N° 1 cylinder must be at T.D.C. ( the N° 1 "T" mark on the governor aligned with the stationary timing mark).

(procedure from the YAMAHA workshop service manual)




XS family is great and worldwide


Patrick

[Toolmaan]

Is there any way you can get that governor plate on wrong so it gives to much advance? It seems to me that it only goes on one way. I know someone was working in that area before I got the bike because 2 of the bolts for the cover were missing. Can you get that installed 180 degrees off like a distributor in a car?

[Toolmaan]

Ok I got the bike running again. I pulled the governor plate off and moved the center piece the one that is oblong, opposite of where it was. Now the bike runs but not well. Hard to start, had to move the timing plate or governore plate all the way to the right, won't go over 1500 rpm. I pulled the 3 srcews off of the plate and held it on by hand. Started engine and moved it more to the right past the screw holes it started to run beter, would accell quick but wouldn't go over 2500 rpm., and pops through carb. It does idle ok, I rigged up some spacers to hold the plate in that position and drove the bike around the block to make sure the trans and rear drive were ok. Something is wrong, I shouldn't have to move that governor plate so far to the right that the screw holes don't line up. Any ideas? Thanks for all the help.

[Geezer]

Pull the choke just about when it starts to falter and if the power picks up then clean the carbs again. I'd bet there are some plugged passeges. Use some of that cheap spray carb cleaner, it's good for see if passeges are plugged or not and sometimes it'll clean the crud out.

Geezer

[Toolmaan]

I got the bike running. I found out that the advance unit that goes behind the ignition governor, was loose on it's shaft and flopping around, the bushing that is supposed to go inside it was gone. I installed a used one I bought off of ebay, Now the bike runs great. Thanks for all the help.

[Bill__Woods]

Sounded like a timing problem more than a fuel problem. If your timing don't advance it will not rev correctly. Glad you got it fixed. The plug caps I am referring to are NGK resistor caps that go on the ends of the ignition wire and connects to the spark plug, get 3 new ones (under $10.00) for good measure.
Long live your triple!

Here is a link to Bob Ryan's Triple Site

[xs1100yam]

OK fine !

Good news

I was also thinking about a similar problem (and similar simptoms), that occur on XS 11 :
On the 2H9 (earlier) and 5K7 (later), the CDI unit look the same, but they are different and not interchangeable (or switchable ?)
With the 2H9 type, advance is mechanical (governor plate) and the 5K7 have electronic advance.
Of course, using a wrong type give either too much advance or no advance at all
But I don't know if the triples have different CDI

I apologize for my poor english, I hope you understand ???

Patrick