Carb alternatives//I'm tired....

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Jul 15, 2005

I've got a 74 850 running stock Amals, both reworked by AMR (Mikuni slides, clean pilots, etc.). The float bowls keep warping to the point where they will leak gas. I'll pull them off, flatten the flange (checked on glass) remount and they will hold for a while.... I get about 1/2 a riding season on a carb before it needs to be done again. I SWEAR I'm not overtightening! Last weekend one of the float bowl bolts let go (stripped the threads in the carb body) leaving me on the side of the road, gas everywhere AGAIN!

So I'm thinking about alternatives. My bikes a bit of a hot rod so I don't want to give up anything. I run 4-6K RPM but also want good throttle response. Anybody have a good recommendation. I'm leaning away from a single Mikuni, from what I hear they slow you down. I do like Dellorto PHFs. Can a any single carb be made to perform as well as the dual Amals (Keihn, Dellorto, etc). I know this has been talked to death but any help would be appreciated.

Or... if someone could just tell me how to keep the fu@#^#% float bowls from spilling!

Oh, another carb that sounds interesting is the Lectron, has anybody tried it....

I run twin 34mm Mk2 Amals not much trouble at all.
carb alternatives

Mk11 Amal, Steve Maney or reworked bevel drive manifolds with 32mm Dell'Ortos, 36w/active pumps if your "hot rod" has any performance enhancing attributes, such as a Combat with reworked head, etc. Amal rubbers will work but you will have to heat the carb side up to fit. You really need the "S" central oil tank with reversed battery location, velocity stacks and K&N's for best results. Would send a photo but am unsure on how.
Thanks for the replies,

Just for clarification, the bikes not all that hot.... its a 74 850 stock head (small port), 10:1 pistons and a Johnson J360 cam. AMR modded Amals w/K&N and peashooters. Trying to maximize port velocity with the small ports so the intake manifold may be a very important consideration.
I like the 74 tapered manifolds.

Dellortos with pumps work well on Commandos but finding the jetting by trial and error is a long business. I can ask around if you go down this route.

The only time that I have had float bowl warping problems was after removing them whilst everything was hot. I assume the temperature was a factor. Are you using the correct (2 BA as I recall) screws ? Whose gaskets ?

If the threads were in good condition and you're stripping then I think that you must be overtightening.

The screws I'm using are the British thread stainless Allens provided by AMR. I have been using regular paper gaskets both stock and some that I made out of gasket material. I also tried some silicone gaskets provided by Rocky Point Cycle which is what I had in when the carb stripped. These gaskets were thicker than the paper so maybe that had something to do with it. I still don't think I'm overtightening them.....

OK, so will PHFs/MKIIs fit on the stock manifolds (width wise) if I just machine off the mount flange and replace it with a rubber bit?

I'm still wondering if any single set up will work as well. A pumper type TM, Edelbrock, Lectron or single Dellorto? or would the single carb/split manifold cause a loss of performance regardless.

Joe, Sorry if I'm telling you what you already know.

The correct gaskets are a composition material. I don't think that you'll get a seal on those crudely-machined, ill-supported faces with paper gaskets.

I've never tried silicone so can't comment there. I use Allen screws and fit them with a ball-ended screwdriver. They shouldn't really need to be pulled up tighter than would be obtained with a normal screwdriver. My feeling would be that hauling on an Allen key would be too tight.
you cannot use your standard manifolds machined down ,to fit dellorto,s And you will gain nothing by fitting them Yes I have done it and it is a lot of time wasted. I have fitted reworked amals mk1 ,done by Bratby .and they are better then new and do not go out of synchro .Anyway if you cannot adjust twin carbs go out and buy a jap
A reason your carbs are bent is that you might be a little bit hamfisted as they distord ever so easily You can flatten the joints on emmery paper and use std gaskets .never had a problem with that ,but then again my wife says I have a soft tough....
sorry to be here again but I just realised I had a similar problem with a set of stainless bolts for a bike I rebuild the bolts wich kept the head to the cases kept becoming undone with obvious resuts ...Try the old steel bolts suplied with the carbs and do not overtighn neither the floatbowl bolts or the nuts with hold the flanges to the head.Overstressing these do bent the carb no end.As I have used std Amals both on 750 and 850 and a racing outfit (that was a real shaker being ridgidly mounted)I cannot understand the carbs are at fault
I put 39mm Keihin flatslide FCRs on my 850, and I love them. I had put a pair on my Ducati 900SS and they positively transformed that bike, so I was predisposed to them. My 850 ran for about a minute with the Amals when I first got it, then wouldn't start again, so I don't have a great basis for comparison, but still I think the Keihins were a big step in the right direction based on what I hear about other alternatives.

Only downsides, beyond "non-stock" complaints, are cost - four figure$$ from CNW, perhaps a bit less from other sources, but I wanted to be able to call Matt Rambow if any problems (there weren't), and a HELL OF A TIGHT FIT. I didn't think they were gonna fit - no way. But I shoehorned them on. Have to hold left sidecover on with a cable tie (no room for dzus fastener anymore), and they do contact my right side petcock somewhat. Also, the air filter box, with attached ignition switch bracket has to go (substitute available from Norvil, as I understand it).

Now that I think of it, I've also had to rejigger my battery holddown arrangements, but I think that's because I put a huge electric-start-capable battery in there (which the bike sure doesn't need - this mod I regret and will probably reverse at some point) - not remembering which particular mod led to that issue. Readily resolvable, in any event (bungee cord!)
You asked the good question about performance with single carb fitted, this is a very relevant issue with getting a wider choice of proven carbs suitable to the Commando.

Historically, just about every British twin particulary Triumph and BSA 650's in twin and single carb form, had NO discernable difference in acceleration and a tiny difference in top speed.

If your 850 has stock or mild cams, and stock pistons then there is going to be very little performance difference between twin Amals and any one of the popular single conversions.

The 73 - 74 850's had the RH10 head, 32mm at the carb down to 30 at the head port. I use a 34mm Mikuni and am compeletely satsified with its starrting and running reliability and overall smooth peformance over the entire rev range. You can fit a 36mm which increases top end power, there is a substantial gas velocity into the 30mm head.

The other advantage of the Mikuni and most single conversions is that they generally fit well, usually straight from the kit. And no bloody road side leaks and coughs and farts like the old Amal. The purist will buy a new pair of Concentrics every 10,000 miles, whereas for the same money you get a dead reliable good performer in the Mikuni.

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