Topic: Restart after rebuild

[schmidbc]

I'm having trouble restarting my '76 CB750F after a top end rebuild.   Could anyone offer suggestions?

I replaced all of the top end gaskets and added new pistion rings, Dyna S ignition and coils, new (Dyna) plug wires, and NGK iridium spark plugs.  The carbs have been throughly cleaned, and reassembled by a compotent shop.  I've set the static timing as per the dyna instructions and checked the plugs to ensure they spark...which they do.  I've also checked and reset the tapped height.

What happens when I thumb the starter is the starter motor spins the engine and I get popping noises and faint puffs of exhaust but that's it!  Can anyone help?

[Patrick]

Wow, where to begin... You need three things to make your engine fire: fuel, compression and a powerful enough spark delivered at the right time. If you have correctly reassembled your top end, taing care the get all the timing marks exactly right when you put it back together, and if you have properly installed the new Dyna system, and if you have properly set the gaps on the valves - all things that no one here can possibly know based on the information you have provided - then spark and compression should be good to go. For the sake of this discussion, I will assume that was all done correctly. That leads to the carbs. You say the carbs were cleaned and assembled by a competent shop. You do not say if the bike ran on these carbs before the top end rebuild or whether the competent show set the carbs up to run on this bike. Almost all of the problems I had getting my bike to run after I restored it tracked back to the carbs. Before you installed them on the bike, I hope you checked the float height. If the float heights aren't correct then the carbs could be getting too much gas or little to none at alll. Were the carbs benched-synched before you put them on? If the sync is too far off the bike may not start. Are the carbs getting fuel from the tank? I won't even ask if you opened the petcock, but did you make sure the gas flowed freely through the gas line into the carbs?

You have to eliminate one potential problem at a time. Whatever is left provides an avenue to te solution. If you are confiident in your mechanical skills and sure that you assembled the tope end and electrical system correctly, look closely at your carbs.

Good luck. When you figure it out it will be well worth the trouble.

[Matt at PSB]

What happens if you switch the blue & yellow wire from the pionts where they plug into the bike's wiring up by the coils. Right now you have "A" plugged into A & "B" plugged into B. What happens if you try A plugged into B & B plugged into A?

Matt at PSB

[schmidbc]

To respond to Patrick first, no the carbs didn't run when I sent them  to the shop, the bike had been sitting for nearly 10 years with gas in them so they had to be ultrasonicly cleaned.  The mechanic I use knew the project and has bench synced carbs for me before, with great results.

The top end rebuild was done properly and all parts of the ignition system pass the tests set by Dyna in their instructions to ensure proper installation.  I agree that my problem probably lies with the carbs.  I'm using an auxlillary fuel tank so I can meter the amount of fuel getting to the cabs.  Right now that's not much.  Do the float bowls need to be primed on these bikes?  My best "educated"
guess is that I'm having a fuel supply, or fuel/air mix problem.

As for Matt... what will switing the coil wires do on dual output coils when the static timing is properly set?  I've never heard that suggested before.  And is PSB PowerSports Business?

Thanks Gentlemen!

[Patrick]

Your float bowls don't have to be primed but you do have to make sure the floats are allowing gas into the bowls. Loosen the drain plugs from all four carbs and make sure you get gas from all of them. Should be fairly free flowing and even. If not, first tap on the bowls in case the floats are just stuck a little. If still no gas, pop off the bowls (turn off the gas first) and make sure your floats move freely. If the floats move freely and all are opening to the same depth, then I think you are going to have to pull the carbs and set the float height. They could be cutting off the gas flow too soon and preventing the bowl from filling. You also say the carb shop knew the project, but did he have the specs? Do you know if this bike EVER ran with these carbs, even 10 years ago? If the floats are set right and the carbs are getting gas, you'll need to check the jetting or the needle height. Running a bit rich or lean won't stop the bike from running -  mine, in fact, ran at both conditions during my "Great Carb Odyssey of Discovery" last winter. If they are way off, though, that could pose some issues. My best guess, if it is the carbs - again, trusting your mechnical ability -  just aren't getting enough gas - evenly - to pass to the cylinders. Try that, see if any of it helps.

[Patrick]

Oh yeah, if you get to the point where you have to pull the bowls, while you are checking whether your floats move freely, also make sure the valve needle is moving with the floats. You don't say how much time passed between when the carbs were cleaned and when they were mounted. If they sat any lenght of time those valve needles might be a little sticky. If tapping the carbs didn't shake them loose, you might have to remove the floats and loosen them up.

[schmidbc]

Yes, these are the original carbs for the bike and they ran "fine" when they were last used according to the previous owner.  As far as the shop goes it's Western Hills Honda.... (www.westernhillshonda.com).  These guys are a great resource for restoration projects or just vintage parts.  Yes, they know this bike.

I do get fuel from the drain plugs but I don't get a free flow really.  I think I'm going to drain the bowls and pull them to check the floats.  Even If I do have to pull the carbs after everything else I've done this shouldn't be too bad.  Thanks for the advice, I'm going to preceed as your perscribed!

[Robert]

What happens when I thumb the starter is the starter motor spins the engine and I get popping noises and faint puffs of exhaust but that's it!  Can anyone help?

I had the same symptoms after top end rebuild....the camshaft was off. Just a thought.

[schmidbc]

How did you correct that?  Pull the motor?  I'm looking at the carbs (off the bike) and everything with the floats seems to be fine.

[Matt at PSB]

PSB stands for Performance Sportbikes.

Why not try what I have suggested with the yellow & blue wires from the Dyna points eliminator plate? Switch them from their current plug to the opposite plug. 4 minutes work & nothing to loose. I’ld love to hear about the results.

Another possible problem if the above does not do the trick is that you do not have a good power connection or not enough voltage on the Dyna plate's power supply wire.

Matt at PSB

[Robert]

Matt is right. Try easy things first.

For the camshaft, _if_ you're not sure that the camshaft is mounted correctly, before you pull the motor (if that is necessary to get to the camshaft ) make sure it is off.

For 750 sohc the intake valve opens 5° before TDC.Ii don't know, maybe use a feeler gauge a tad thinner than valve clearance between valve and rocker and by turning the motor by hand try to detect the point where you can't move the feeler gauge anymore, should be 5° before TDC.

Maybe someone has a better trick to do this.

[schmidbc]

I'll try the coil swap next if this doesn't work, I'm still not sure of the theory behind it though.

I think what I've determined after taking a look at the carbs, rechecking my Dyna installation, and looking over the proceedure that I used to install the camshaft is that I've installed the camshaft as per the Clymer manual, albiet incorrectly.

All that the clymer maual says is that when you have the 1-4 cylider at the "T" mark for top dead center you should put the keyhole on the cams at 12:00.  What it does not say is to make sure that you're on the compression stroke and not the exhaust stroke.  Does this make any sense or am I just overthinking my problem? 

Basically what I believe is that I've installed the cam correctly as per the instructions laid out by clymer and come off by 180 degrees.  I belive that this explains the small puffs of exhaust that are coming out of the muffler, the fuel is igniting as it's leaving the chamber rather than when it's being compressed.

This is a fine theory in my head, but I'd love to hear everyone elses thoughts on it too to make sure I'm not crackers.

[Clyde]

What Matt is getting at with the wires, is that 1 and 4 fire together every revolution of the crank and 2 and 3 fire every revolution but 180 degrees apart. If you have the wires wrong or have assembled the auto advance incorrectly (as I did once 15 years ago) you will only get an occasional pop from your exhausts as the spark plugs are firing when the crank is at the bottom of the stroke not just before the top.
 

[schmidbc]

Oh my, did they revise the Clymer again?  My Clymer, ninth printing 1990, does show the correct installation procedure where the notch on the cam is at 3 oclock and in alignment with the horizontal surface of the cylinder head, viewed from the right.  Further, it shows one lobe at the 10 o clock position, presumeably the lobe at the right most position.  The cam determines whether it is a compression stroke or what ever.  It turns half as fast as the crank.  I seems you put the cam in 90 degrees off.  I wonder if you can bend valves this way?  I'm not trying it.

Ouch TwoTired.....ouch.  Am I getting outdated shop manuals at a vintage parts shop?

Well, my Clymer Eighth Edition Seventeenth Printing (July 1995) says this:
Chapter Four Engine:  Camshaft, Assembly

4. Time the valves by setting the No. 1 and No. 4 cylinders to top dead center (TDC) by aligning the "T 1-4" mark on the spark advance under the preaker points with the index mark.

5.  Put the sproket on the camshaft, and run the chain through the right side.

6.  Put the camshaft in the holder and align the timing line on the shaft end, so it is parallel with the horizontal surface of the holder.  The key groove should be at 12 o'clock.

7.  Mount the sprocket and cam chain on the shaft with the 2 bolts as shown in Figure 45.

etc..............................

Please outline for me where this is incorrect, I don't want any surprises after this much work.

[TwoTired]

Opps, sorry, I didn't go back in the thread far enough.  I thought you had a CB550 not CB750.

My bad...

[schmidbc]

I'm begining to think that there's a discrepancy in the installation between the 550's and the 750F.  Here's the same picture from the 750 manual.

Let's assumre for a minute that I'm following the manual properly, is my theory about the ignition firing on exhaust stroke rather than the compression stroke a possiblity?

[schmidbc]

Thats oK!!!!  I'm always glad to get enthuastic responses..........I must admit though that you had me worried for a minute...bent valves.......!

[TwoTired]

Let's assumre for a minute that I'm following the manual properly, is my theory about the ignition firing on exhaust stroke rather than the compression stroke a possiblity?

Well if I have any credibility left at all (mmm...good crackers).  There are 4 cycles: intake, compression, power, exhaust.  Spark should happen between compression and exhaust.  If indeed the 750 spark advancer cam can be assembled 180 out and put on the crankshaft like the 550 can (<---IF), then you would get your spark between the exhaust and intake strokes.

If you put the cam in correctly in relation to the crankshaft timing marks, IT will determine whether it is an exhaust or compression stroke, etc. by opening and closing the appropriate valves.  It turns half as fast as the crankshaft.

I'm going to go crawl away now... (hic)

[dzachary99]

I just went through this same situation about two months ago, and it was my coil wires. I had 1-4 hooked to 2-3 and so on. I'm laughing to myself because I lost a few hours of sleep thinking I had timed the cam wrong. If you haven't tried switching the ignition wires, I would.

Good Luck,
Doug

[scondon]

Try spraying carb cleaner or starter fluid directly into the air intake of each carb and then hit the starter button.  If bike runs like a champ for 1 or 2 seconds then I would concentrate on the carbs. If it exhibits the same symptoms(wont start) then I'd troubleshoot the ignition. Two Tired's right, the cam will determine intake or exhaust stroke so your install should be OK.

[schmidbc]

Thanks to Matt and Dzachary for the help on the plug wires.  I followed what Matt was saying and kind of came to my own conclusion that I had performed the install on the coil wires wrong.  Too late to try to fire it up tonight, but I'll try first thing in the morning.

Here's my thought process....

So I was using a 12v timing light to set the static timing on the bike.  The instructions that came with the Dyna ignition said to ground the light, and hook it up to the intersection of the blue wires near the coils....which I did.  The telling part is that the instructions said to then set the crankshaft to the advanced timing marks for the 1-4 cylinders.  Get it?  Blue wires = 1-4.

At that point I looked down a bit to see what wires I had coming out of the coils the blue (1-4) wires were going into.  And...you guessed it dzachary99 and Matt at PSB....the plug wires were going to 2-3.  So, I swapped the plug wires so the blue wires were going into the coils that the 1-4 plug wires are coming out of.  And the yellow wires are going into the coils that the 2-3 plug wires are coming out of.

I hope that everyone understands this so that you see the clear example of how stupid you look when everyone is telling you the answer and you won't listen.  Learn from my stupidity newbies.

I'll leave a report tomorrow after I attempt to fire it up.

[schmidbc]

Gentlemen, she's running and she starts with just a very quick thumb of the starter.  Thanks again Jeff and zachary99 for putting me on the trail.