CB750 1981 DOHC BUILD - ISSUE WITH STARTING / FIRING

BrandonP

New Member
Hey Guys,

I'm in the middle of a rebuilding a CB750 1981;
This is my first ever build and It's been a whirlwind of forums, google, youtube and a local motorcycle shop to get me on track to this point.

However, I've hit a bit of a snag; the bike wont start / fire, and I wondered if any of you can help out with some advice...

There's a few issues -

- The fuel diaphragm valve doesn't appear to be working great, but in a bid to start the bike for now I bypassed this to fill the carbs with fuel and try turn the bike over. Once I know we're up and running properly I'm going to try get this sorted and ideally keep it working on the bike - I fear I'll forget to turn the Fuel Petcock OFF one day if not...

- Following the bypass, the float bowls definitely have fuel, but after turning it over a few times I checked the spark plugs (which are brand new) and they were bone dry.

- The spark plugs are sparking (tested against the engine casing), so I assume my wiring is correct to the coils and they are obviously working as they should be, right?!

- Also do the CDI / Spark Unit Boxes have a particular order when they're wired up - there doesn't seem to be an outline for that online anywhere and the boxes aren't marked by number or anything.

- I've removed the old air box and now have Pod Filters on the carbs - now I know there's usually issues on this front and people say avoid it in favour of the old Air Box, but what I'd read up on this seemed to really only affect the bike bogging down etc when riding, not actually issues with starting. Would it affect start up?

- When I previously took the carbs apart I changed the Main Jet and Idle Jet in the middle of the carbs, plus the gaskets, but nothing else. Should I have adjusted my Pilot Screw or anything now that the jets have changed?
(Overall, the carbs seemed pretty clean to me, but as a rookie I'm not sure what I'd be looking for otherwise or how to fix any issues I'd come across).

- Lastly, any other ideas on what would be preventing fuel making it to my Spark Plugs?!

Really hope you guys can help...

Thanks,
BP
 

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Did the bike run before?

How are you providing fuel to the carbs with the tank off? How did you "fill the bowls"?

Have you checked/set the carb float bowl heights?

What jet sizes did you install?

Have the valve clearances been checked and/or adjusted?
 
Hey Canyon,

1) This sounds crazy now it's come to not starting, but...
I bought the bike via an import eBay shop that takes bikes from the USA and ships them to the UK.
Most don't start and they advertise as such, so when purchasing I stated I'd only take something that starts and runs.
They assured me it did (and I do trust that), but when it arrived to me via courier the battery had run to almost empty - so tried to start up and it took a second or two, popped and spluttered into action, seemed to rev really high and then drop low and cut out. Then the battery was straight empty.
I didn't know why (rookie) and got caught up in my bike build dreams I started taking the thing apart (and there's the rookie mistake). I did spot when removing plastics however the Carbs were only half connected and half hanging loose - later also finding the petcock barely let any fuel through. So I'd sort of put it down to those two issues causing what went down. Now though (Carbs fully connected and a new petcock installed), I'm starting to think I'm a bit wrong on this one...

2) Provided fuel to the Carbs with the tank being connected and on the bike - to bypass the fuel diaphragm valve I just popped out the spring (forum member told me this would work for what I was after). I know they then filled with fuel as when the petcock was flipped to ON they quickly filled, then quickly started to overflow and drain out. Given I was bypassing the valve I didn't think much at the time, but now reading more, this may signal my floats have an issue. Is that correct? Also, I turned the petcock back to off and the fuel in the carbs remained to try start the bike.

3) I guess I kind of answered that one above - I didn't. I wasn't sure I even wanted to open the carbs myself originally but after a quote from a local workshop I thought I'd at least give the jets a try. After that though, I didn't realise there was other work that could and should have been done! How do I go about checking / setting the height?

4) They were 68 / 102 > After a lot of reading forum posts, I changed up to 72 / 118.
I've added pods and a 4-1 MotoGP Werks exhaust, so going by various calculations those were the sizes recommended.

5) They haven't. Again, figure this is a rookie error and they should be.
However, this involves a lot more mechanical work than I really feel comfortable with and worry I'll make things worse.

Thanks for responding and trying to help out!
BP
 
Ok. Good info. Everybody gets ahead of themselves with bike builds.

If your carbs pissed fuel, then you'll need to open them.
There are tons of videos and articles on the web that can explain your carbs better than this post.
Those carbs aren't too bad but you will definitely need to understand them to own and run them.
The basics are you need a good seal between the float needle and the needle seats to prevent
the fuel from pissing past the seat and going all over. You also have to set the floats at the correct
height in the bowl, it's easy, lots of videos.

The rubber carb holders are also very important. If they look like crap, get new ones. They MUST seal correctly from the carb to the engine.

What brand of pod filter did you use? If they were the cheapo Emgo's, take them off and don't use them. If they are K&N, you're ok.
 
Hey Brandon, welcome mate. Just wanted to tell you not to get daunted by any of this. It's all totally doable! Trust me - 2 years ago when I got hold of my '81 CB750 I didn't know my ass from an angle grinder but with the help of this forum and a fair amount of blood, sweat and beers I ended up with a pretty decent ride. Here's a link to my thread if you're interested - http://www.dotheton.com/forum/index.php?topic=72170.0 It's a bit of a read to put it mildly, the thing goes on for nearly 100 pages. But there's a ton of good info there that may help you too.

Don't be nervous about pulling the carbs and getting stuck in. Google Mike Nixon - he's a carb guru with a very helpful website and booklets on these carbs available for purchase for a few bucks. Indispensable. I kept the stock airbox on mine so didn't have to dick about too much with the jets. And doing a valve check is a piece of cake too. You got this!

Good luck man, great looking ride you got there!
 
Hi Guys,

@CanyonCarver -
Thanks for the guidance mate.
Opened the Carbs again to check float needles and floats themselves - Pics attached.

They seem correct and in good working order - they did only overflow when I bypassed the vacuum fuel valve and turned the tank petcock to ON; so figure there isn't actually an issue with this area when that valve works correctly.

I did spot there seemed to be an ever-so-slight amount of leaking fuel on the outside of the gasket seals at the float bowls. This wasn't enough to drip or anything, but was damp to touch. I'd replaced those gaskets with brand new ones previously, but they are not like the originals which sat in grooves, they covered the full area. As this is different than standard, I'm going to replace these again with the original style and see if that fixes that issue.

I also checked the vacuum valve is unblocked - which it is.
The rubber carb holders also appear to be in good condition, they provide a tight seal and are not cracked - again, picture attached.

I did wonder; if all of this happens to be in decent enough condition, would the issue be as simple as a fuel / air mix?
Particularly as the new air pods will have changed the requirements for this compared to when the original air box was on the bike?

@Jimbonaut
Thanks for the welcome and the words of encouragement...
I started this build months ago with so much enthusiasm.
However, issue-by-issue and cost-by-cost that's drained some from back then and I really just need and want this to actually go to plan. I guess anything worth having doesn't come easy though, right!?

Thanks for the link to your thread - it's going to take me some time to get all the way through, but it's already a great read and your bike looks AWESOME.

Thanks on the link to Mike Nixon too - I think this is definitely something I'l invest in if my issues don't work themselves out in the next week or so.

Will let you know how it goes....

______________

IF ANYONE ELSE CAN OFFER ADVICE, I'D REALLY APPRECIATE IT!
 

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I'm no carb guru by any means (literally - by no means at all) but yeah, putting pods on instead of the stock airbox is definitely gonna mess with the carbs. They alter the amount of air going into your carb and therefore you need to adjust the amount of fuel. This in turn means mucking about with the jets (and/or other parts of the carb which, hopefully, other people will be able to help you with) in order to dial down the mix. Apparently these carbs are tricky to harmonize with pods, but it's doable with a bit of time a patience.

And those float bowl gaskets definitely don't look right (to me) - the proper ones that sit in the grooves of the bowl should stop the leak. Good luck man - keep asking questions here and you'll get the answers. Worked for me 8)
 
did you take the main jet holder and main jet out of the 4th picture down or is it just missing?
 
@Jimbonaut
Time I have, patience not so much.. I see by getting my self 50 pages deep in your thread you follow that too!
Yeah those float gaskets seem to have warped themselves some by getting wet with fuel too, so hopefully going back tom OEM style fixes the issue.

I spotted a few things in your thread I'm keen to drop you a PM on, so will get to that soon...

For now though, I've taken advice in your thread that seemed to work well and bought myself a compression tester.
I'll give this a go when it arrives and post my findings, just to double check this is inline with requirements for that part of things.

For clarity; I spotted you used some kind of cylinder spray -
1) what did you use?
2) do you spray this in the rubber boots directly to the motor where the carbs would normally connect?
3) if so to the above - is this before turning the motor or during?

Also, I got the part about keeping the throttle wide open and all plugs out - I'll probably use youtube when it comes to it to keep me right.

@Cxman
Sorry yeah, should've said - I'd only popped it out to take a look everything seemed clear underneath.
When I'd taken the entire Main Jet out of the furthest right side carb (from being sat on the bike), it did appear to have some sort of dirt on it - which I assume is old fuel? (pic attached).
 

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Hey Sonrier,

What do you mean bypassing the carbs?!
Sorry, newbie question again...

I tried to fire the bike with carbs off and plugs out just to check it had compression - it did, but hopefully running a proper test will give a better indication of where I stand on this.

Thanks,
Brandon
 
Sonreir said:
Have you tried bypassing the carbs and seeing if the bike will fire on ether?

That's not a bad idea "ether". Ba dum tiss...

Brandon, it still sounds like carbs.
 
BrandonP said:
Hey Sonrier,

What do you mean bypassing the carbs?!
Sorry, newbie question again...

I tried to fire the bike with carbs off and plugs out just to check it had compression - it did, but hopefully running a proper test will give a better indication of where I stand on this.

Thanks,
Brandon

Starting fluid (aka ether) can be used in place of petrol to get the bike to run for a few seconds and it's a lot less picky about air ratios. If the engine does run, then you've positively identified the problem as being with the carbs. If the engine doesn't run, you're probably looking at a spark/timing issue, or maybe compression.
 
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