Single Carb Conversion - Brosol Solex H30/31 - Need Help

[adamhixon]

Hi all, this is my first post here but I have been really enjoying this forum. There is tons of good information here.

I'm having some trouble with my attempt at a single carb conversion on my '76 GL1000.

History:
The bike ran and was "ride-able" (sort of) with the stock carbs but they were in rough shape. They are in rather desperate need of a full-rebuild with classic symptoms like poor starting, would not idle below 2.2k, smoke, backfire, poor throttle response etc. I decided to go with a single carb for the simplicity of maintaining it long term and of course price.


Current:
The bike has the dyna electronic ignition installed but is otherwise stock.

Its a homemade manifold with the Solex Brosol H30/31 carb installed.
- Main jet is currently 112.5
- Idle jet is currently 050
With this conversion, it starts smooth every time and neutral throttle response is excellent. (Honestly better than I had hoped)
It seems to have eliminated any unpleasant engine noise.

The Problem:
It has no torque under load at all and I still can't seem to be able to get it to idle smoothly under about 1.7k.

I'v been working on it in my fathers garage which is a few hours away from where I'm living now so work is slow going. (about one weekend a month) For my next trip home I'm going to re-check the engine timing as I did install a new timing belt recently, however I'm pretty sure its correct.

I'm really not much of a mechanic, so I would appreciate any ideas at all. Thanks.

Manifold.jpg

SingleCarb1.jpg

PS: I had tried to post this under the Unique Carburation section, but I could not find the "new topic" button there. Is it not open for new posts?

[sunnbobb]

Very difficult to say given it was running poorly with the stock carbs. Also, the problem may be inherent to the design of the single carb plenum. I also wonder if that plenum should not have a blow off valve. A back fire could set off a bomb.

[fish]

Great work Adam
adjust your valves.
Tight valves causes all kinds of problemes that are frequently mis diagnosed as carburation or ignition

[robin1731]

Most everyone with the single carb has talked about having to advance the timing to get it to run better. That doesn't explain not being able to get to idle below 1700rpm though.

What is the brass valve looking piece with what looks like a rubber plug? Is that for a vacuum source? If so that could work as a pop-off valve too. I think.

[adamhixon]

A back fire could set off a bomb. - sunnbobb

OK, You have my attention. Is this a risk of any single carb conversion or is there something specific in my design that worries you?

And yes, that capped brass fitting is a vacuum source. I'm told it is not necessary with this carb, but I wasn't sure when I was building this thing so I put one in there. Perhaps this could function as my blow off valve if needed.

I did adjust the ignition timing a bit after this install and settled on a little advanced. I will check it again. I will also add a valve adjustment to my list for my next trip home.

I am a bit worried that it is just a poor design. Its all hollow in there, nothing going on that you can't see in the pictures. That is a lot of volume compared to many of the other designs I have seen around the web. I assumed that would give me less than ideal throttle response, but might it cause my no power under load issues? Hmm

Another interesting quirk, when it is running on the center stand, the motion of pushing off of it kills the engine, every time so far. I assume this has something to do with fuel in the bowl, but I'm at a loss as to why. If that problem ends there I'm ok with it, I just hope it doesn't turn into issues when hard breaking or cornering.

Thanks for the tips.

[fish]

I am a bit worried that it is just a poor design. Its all hollow in there, nothing going on that you can't see in the pictures. That is a lot of volume compared to many of the other designs I have seen around the web. [/quote]

It would seem to me that excessive internal volume might affect response but I do not see why it would affect steady state cruise at a steady speed

[fish]

Another interesting quirk, when it is running on the center stand, the motion of pushing off of it kills the engine, every time so far. I assume this has something to do with fuel in the bowl, but I'm at a loss as to why. If that problem ends there I'm ok with it, I just hope it doesn't turn into issues when hard breaking or cornering.

Thanks for the tips.[/quote]

In my opinion this has to be an electrical issue
I bet a loose ground or maybe a battery cable that gets rattled by the thump when the centerstand retracts.That centerstand really whacks the bottom of the bike when you roll it off it.
Tie a wad of rags around the centerstand legs to cushion the blow & try it again...

[Sagebrush]

Moved to unique carburation forum.

[Fred Camper]

The large plenum means low velocity and that may be an issue due to puddling fuel. But and intake manifold designer I am not lol.

[gregforesi]

There are a number of single carb conversions around here that worked out pretty darn well. Take a look at the manifold on HOTT's "Ole Sparky".

This is an opinion (and opinions can be subject to argument) but here goes...
Remember that you are replacing (4) 33mm carbs with a single 25.5mm carb. The stock carbs provide a total of 3422sq-mm of carb throat area and the Brosol is about 510sq-mm. You're about 15% of the stock carb throat area. I would expect that the engine would struggle to hit the redline as it works to pull enough air through that particular carb. The vacuum signal would be large, and the if the air corrector isn't dialed in, it would go real rich at steady speeds (near the top end) as well. I don't think it would be making many horses at high RPM either.

I haven't seen anyone do this...I think I'd go with a Holley 390cfm 4bbl as a single carb. (Unless you overdue it - breathing is good).

[fish]

There are a number of single carb conversions around here that worked out pretty darn well. Take a look at the manifold on HOTT's "Ole Sparky".

This is an opinion (and opinions can be subject to argument) but here goes...
Remember that you are replacing (4) 33mm carbs with a single 25.5mm carb. The stock carbs provide a total of 3422sq-mm of carb throat area and the Brosol is about 510sq-mm. You're about 15% of the stock carb throat area. I would expect that the engine would struggle to hit the redline as it works to pull enough air through that particular carb. The vacuum signal would be large, and the if the air corrector isn't dialed in, it would go real rich at steady speeds (near the top end) as well. I don't think it would be making many horses at high RPM either.

I haven't seen anyone do this...I think I'd go with a Holley 390cfm 4bbl as a single carb. (Unless you overdue it - breathing is good).

Greg I agree with you.
If I agree with you that is the same as if you agree with me
If you agree with me you must be right

I think the bottom line is:
: getting a single carb setup to perffom as well as the stock set up would be more work than fixing the stock setup
:The only way a single is preferable is if you are willing to settle for much less performance.

[ericheath]

Unless you're feeling lucky. Well, are ya? Are ya feeling lucky.

[WShie]

Fish wrote:[quote][/quote]I think the bottom line is:
: getting a single carb setup to perffom as well as the stock set up would be more work than fixing the stock setup
:The only way a single is preferable is if you are willing to settle for much less performance.
+1 Well put!

[badorderbob]

So what is the out come or decision,... remake another manifold, rebuild Oems?

[adamhixon]

Well,, I'v got the next couple of days off work so I am considering a trip home tomorrow to tinker with it. I'll re-check the engine timing and valve adjustment, ignition timing and electrical as suggested here. And anything else I can think of. If I still come up short, I will probably end up re-building the manifold. I have looked at Ole Sparky's design and it seems easy enough to duplicate. I should have gone with something like that in the first place, but this one went together in a day in the shop with materials at hand. I don't really have any interest in re-building the OEMs, as I like the uniqueness of the single carb. I know I'll never get the performance out of a single carb that I could out of well tuned and synced OEMs, but I'm really ok with that. I'm of the opinion that I dont need to be red-lining a 36 year old engine with an un-known history anyway. I just want a reliable bike that runs smooth.

Question:

Remember that you are replacing (4) 33mm carbs with a single 25.5mm carb. The stock carbs provide a total of 3422sq-mm of carb throat area and the Brosol is about 510sq-mm.

I thought about that and didn't think it would be that big of a change. To my reasoning, since each intake valve is only open for less than 1/4 of the time and only 1 at a time, your only making use of each 33mm carb less than 1/4 of the time. The solex carb will only be supplying airflow to one carb at a time so the change is really just 33mm to 25.5mm instead of 33mm times 4 to 25.5mm. I really don't know a lot about engines and I have virtually NO practical experience, so is my logic flawed in some way? This whole thing is a learning experience for me, so I'd enjoy any discussion on this.


Just to re-state my original problem, It has no power under load. And I mean none at all. Its not just a performance issue. The engine just dies as I release the clutch, almost as though I was trying to launch in fifth gear. It feels like I could probably make it go If I slip the clutch a ton, but really completely un-ridable.

Thanks again for everyone's support! I'll keep you updated.