Triumph Rat Motorcycle Forums banner

1 - 20 of 21 Posts

·
Registered
Joined
·
424 Posts
Discussion Starter #1
I have to make a decision here before too long. The seller of my project bike was going to change out the original dual carb head with a single carb one that he had (pictures below). He let me take both heads home to decide which direction I wanted to go it and I said I would return the unused head. I done some searching here tonight and found people seem to lean towards the single carb conversion but there are the traditionalists here that are opposed to that. I have two MGs that I run often and one is restored to a pretty high order and essentially original and then there is the other one, (my winter beater) that has taken all kinds of mods over the years. I love them both for different reasons. Maybe early on, the idea of an easier to get dialed in single carb head seems like the far easier direction but both my MGs are running twin SUs and those, once set up and dialed in, they run forever. So, these are the two heads I have to choose from, what choice should I make?
Two shots of the twin head;




The single head;


 

·
Premium Member
Joined
·
4,721 Posts
The Bonneville head doesn't look like '71;there are no locating pins for the rocker boxes.If it's '69/'70 it should still work OK.Check the depth of the pushrod tube recess,and see if it's the same as the other head.Check how a '71 rocker box gasket will fit the head,and the rocker box.It looks OK,but some early heads have less sealing area where you need it.

The manifold on the single carb head looks to be aftermarket,for a different carb.The standard manifold would have 2" bolt centres at the carb flange.

If you can short out a spark plug or plug lead,you can tell if carbs are synchronized.It's not hard to adjust the idle stops or cables,and they won't change in a hurry.You can get more equal mixtures with twin carbs,even if it means different size main jets.
I'd use the Bonneville head.It will also fit the existing standard air filters.You won't have to re-jet,if the carbs are still set up for a Bonneville.

It will still be a proper Bonneville,not a 1/2 breed.
 

·
Registered
Joined
·
4,174 Posts
Twin amals aren't at all difficult to get synchronised and balanced. The trouble is, they don't stay that way for long. When I rode a T140 as daily transport I had to re-do them every week - not a long or difficult task, and satisying to get the engine to that 'spot-on' feeling again, but combined with weekly checks of the points it added up to a bit of an unnecessary-seeming chore.

I opted for a single carb TR7 when recently returning to Triumphs after years with Guzzis. The idea was to reduce routine maintenance to a minimum, so with a single carb and electronic ignition I thought I'd made a sensible choice. But I was forgetting that old Triumphs just aren't ride & ignore bikes and can't be made into one: I had to face that unless I was willing to get back into frequent tinkering and occasional more major work, I'd be better off with a Guzzi!

I wouldn't be over-wary of twin carbs. As Pete says, you can get the mixtures for each cylinder very accurate whereas you're stuck with a compromise with a single carb. Just be prepared to become a carb tuning expert!
 

·
Registered
Joined
·
270 Posts
From a practical standpoint, I suppose it would depend on which rocker boxes you have, the 1971 versions will have the locating holes for the pins on your single carb head. These were added to stop the boxes from 'spitting out' the gasket on the back. Pre 71 version won't have these holes. For sure check the pushrod tube countersink on the bottom of the heads, it can be a lot of fun and games getting the O ring/flat rubber washer/etc right so it doesn't leak like a sieve there.

Aesthetically, I've always preferred the twin carb look, single carbs are a little easier to live with, only one pilot jet to gum up with lousy gas over the winter.
 

·
Registered
Joined
·
424 Posts
Discussion Starter #5
Thanks for the replies. MrPete, The clean head still set up for dual is from teh motor in teh frame. The engine number starts with a KG which would put that as a 9/72 build. It does not have the locating pins but everything else is spot on and the rocker boxes fit well. The other head (diff part numbers) is teh same (to my untrained eye) except that it does have the locating pins. Here are two pics to help illustrate. I definitely understand the "look right" bit. As said before. All three of my MGs have early twin SU carbs on them because that is the way they are supposed to look (plus they are simple to work on and perform well)
Eman, I will recheck in a bit ref the pushrod tube countersinks in teh heads. I have to say, from looks alone, I prefer the individual air filters over teh air boxes which seem a bit ungainly or awkward but they are simple so I will not make decision on that. I guess I will chat with my mechanic about his thoughts since he will be the one I will fall back on when not comfortable with something. I am reassembling the engine at his shop this winter.
The rocker boxes;


The other head;


Part number on that one;
 

·
Registered
Joined
·
270 Posts
The head with the pins and the rocker boxes with the pin holes are set up so you can bolt the head to the lower end and get it all torqued down without the rocker boxes on there to make it easier to get the engine into the OIF frame. There's not much in the way of clearance there. It's a different set of head bolts, the four in the middle have an internal thread so the short ones can bolt the rocker box on afterwards.
Another thing to consider, is the single carb setup has very little space between the carb and the oiltank/downtube. Both versions originally used the airbox setups, if you are dumping the airboxes, you need a REALLY skinny aircleaner for the single.

Those rockerboxes you have there are the 71/early 72 with the access plugs in the side for the feeler gauge.
 

·
Registered
Joined
·
424 Posts
Discussion Starter #7
Eman, Thanks for your time. This is helpful. You are making me lean towards doing things the 'right' way. When I was thinking about separate air filters, I was thinking for the dual carb setup. I think it may (or will) be a PITA at times but I can always change later so I will probably go with the dual setup but I will talk to my mechanic and also get his feedback. He is almost entirely a Brit car mechanic but has worked on and owned a few in the past (he has a VERY nice Hurricane stored in the back of his shop). Are you saying I should stay with the head that has the locating pins for ease of assembly?
Ref engine fasteners, I am planning on going with mostly new stuff for peace of mind (not very expensive from what I have seen).
Thanks again, Carl
 

·
Registered
Joined
·
270 Posts
In the end it's all your decision, anything can be made to work with anything with enough time and money.
If you use the twin carb head, make sure you have the two piece inner four head bolts so you can torque the head on when the engine is in or out of the frame. Engine will be really hard to get in the frame with the rockerboxes on. It's not impossible to get the head and rocker boxes on with the pre-71 long bolts, but it's not much fun at all rocking the head to one side so you can feed the bolt past the huge backbone tube, tough to keep the pushrods in place at the same time. I've done it before, but man, it sucked.

Easiest? Use all the 71 components, best chance of no leaks that way.

But again, your bike, your vision.
 

·
Registered
Joined
·
424 Posts
Discussion Starter #9
Arghhh, this is getting complicated! Now it seems the clean, two carb head which appeared to come with the late '72 engine is actually from a pre unit engine (late 60's maybe). The one that dates closer to the balance of the engine but it is a single carb head. What model did this come from in 71/72? Were there non OIF Triumph 650 twins at that time?
 

·
Registered
Joined
·
270 Posts
The Bonneville head is from a unit Bonneville, likely 1965-1969.

71 head is off a TR6, either a TR6C or a TR6R, engine-wise, they were the same.
There were no non-OIF 650s from 1971 onwards.

What model designation is stamped on the engine?
 

·
Registered
Joined
·
424 Posts
Discussion Starter #11
the bonneville head is from a unit bonneville, likely 1965-1969.

71 head is off a tr6, either a tr6c or a tr6r, engine-wise, they were the same.
There were no non-oif 650s from 1971 onwards.

What model designation is stamped on the engine?
t120r
 

·
Registered
Joined
·
424 Posts
Discussion Starter #12
So running the pre unit head would not work well? To my untrained eye. it seemed identical other than the lack of pins to hold the gaskets.
 

·
Registered
Joined
·
270 Posts
It's not a pre unit head, they only had eight headbolts, unit version have nine.

Neither of those heads is the one that came with that engine... presuming that's the engine that came with the OIF frame.

The twin carb head can be made to work well, just need to see what else you have to work with as far as headbolts etc.
 

·
Registered
Joined
·
424 Posts
Discussion Starter #14
I am going to guess this is what is called a "Bitsa" basket case. It is an early OIF frame with a late 72 engine with the earlier of the two heads on it. It came with two Amal carbs. Sorry about thinking it was a pre unit head. I am still learning the Triumph Motorcycle world :)
Here are the pics of the current engine bolts along with a couple of other shots that may help you (and me)






Heading out to see the seller for more parts found
 

·
Premium Member
Joined
·
4,721 Posts
The T120 head looks like a '70 head.On one side,you have the cast number E3663.On the intake side there should be a cast oval.If that oval has "68" cast in it,then it's a '69 model head.'70 model may have no number there.
The head is OK.Check the depth of the pushrod tube recess,but it should be the same.

The long head bolts you have don't suit OIF.They suit '70 and earlier dry frame,and they aren't suitable for '71 onward rocker boxes.They would be difficult to fit and would cause damage.'71 and '72-onward 650 inner head bolts had allen heads (although different sizes).They have a fat round section inside the rocker box,and a tapped thread on the end above the recessed allen head.Better bolts are available (studs actually),that have 26 tpi to suit the barrel threads and 24 tpi at top to suit T140 allen-head nuts (T140 studs will not fit the barrel).

The barrel you have seems to have correct tappet blocks to suit '69-onward pushrod tubes.
 

·
Registered
Joined
·
143 Posts
While I can't help you for sure identify the heads you have there, I can tell you that dual carbs is not that hard to make work right. And unlike what the one poster says, you do not have to adjust the carbs and points weekly! I have 10 different british bikes, all with dual carbs, and all but 1 with points. They get ridden regularly and one I set them up, I have never had to adjust the carbs and points again. I have 68 Bonneville that gets ridden the most and the points haven't needed adjustment in a little over 5 years, and I have never had to adjust the carbs since I rebuilt the engine 8 years ago.
 

·
Registered
Joined
·
4,174 Posts
While I can't help you for sure identify the heads you have there, I can tell you that dual carbs is not that hard to make work right. And unlike what the one poster says, you do not have to adjust the carbs and points weekly! I have 10 different british bikes, all with dual carbs, and all but 1 with points. They get ridden regularly and one I set them up, I have never had to adjust the carbs and points again. I have 68 Bonneville that gets ridden the most and the points haven't needed adjustment in a little over 5 years, and I have never had to adjust the carbs since I rebuilt the engine 8 years ago.
All I can say is that I'm very surprised. It took me quite a while to realise it, but eventually I did realise that the reason I didn;t get that really 'sweet' feeling when everything is just as it should be, was because everything wasn't quite as it should be! The bike would still run well enough, start first kick, tick over well and go quite nicely - but not REALLY sweetly, not to my feel anyway. But if I checked the carb slides for simultaneous fully open & fully shut (fully open being most important) every week, I almost always had to adjust one cable or the other a little bit, and then that 'spot on' feeling would stay there. Maybe some people aren't as sensitive as others to what feels right?

As for points, I didn't say I needed to adjust them every week, but I did check them at the same time as doing the carbs, and quite often they did need some adjustment. You must've had a Triumph with much better parts than mine...
 

·
Registered
Joined
·
1,123 Posts
Adjusting twin carb's entails an understanding of the various cables also.
Starting from the bottom - you have short cables to an intermediate adjuster.
Moveing up you will find the splitter.
Up again to another adjuster before the handlebar.
Personally, never could get my Amal's right and opted for a set of Mikuni's.
The Mik's have a cable adjustment right on top of the slide that Amal doesn't have.
This is used to establish a bit of 'slack' after the throttle slides are synched for equal opening from idle.
Next up is equal opening at mid or higher throttle position.
Once the slides are synched at a higher setting, the next set of cable adjusters can be set for the same bit of 'slack' as before.
This will establish equality before the splitter.
The following explanation was gleaned from a search to help me set up my carb's from scratch- using all new cables.
No other manual or web search instruction made any sense to me until I stumbled on a site for a Russion Dneper cycle with a K-68 carb.
The following instructions were perfect to allow me to set up my Mik's!




http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=22&cad=rja&ved=0CCgQFjABOBQ&url=http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=255258&ei=pVVvULTgGpC20AH8goGYAw&usg=AFQjCNELRNb9EtjF-gA2xslPZ6jgRhg7Ow
 

·
Banned
Joined
·
1,522 Posts
Adjusting twin carb's entails an understanding of the various cables also.
Starting from the bottom - you have short cables to an intermediate adjuster.
Moveing up you will find the splitter.
Up again to another adjuster before the handlebar.
Personally, never could get my Amal's right and opted for a set of Mikuni's.
The Mik's have a cable adjustment right on top of the slide that Amal doesn't have.
This is used to establish a bit of 'slack' after the throttle slides are synched for equal opening from idle.
Next up is equal opening at mid or higher throttle position.
Once the slides are synched at a higher setting, the next set of cable adjusters can be set for the same bit of 'slack' as before.
This will establish equality before the splitter.
The following explanation was gleaned from a search to help me set up my carb's from scratch- using all new cables.
No other manual or web search instruction made any sense to me until I stumbled on a site for a Russion Dneper cycle with a K-68 carb.
The following instructions were perfect to allow me to set up my Mik's!




http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=22&cad=rja&ved=0CCgQFjABOBQ&url=http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=255258&ei=pVVvULTgGpC20AH8goGYAw&usg=AFQjCNELRNb9EtjF-gA2xslPZ6jgRhg7Ow
Thank you for that presentation. This was kind of my thought to about AMAL's and after we get my bike up and running with the old AMAL's I do have we are looking to go up to the Mikuni's.
 

·
Premium Member
Joined
·
4,721 Posts
Nothing tells you the truth as much as shorting out plug leads,when it comes to carb synchronizing.
Relying on accuracy of slide cutaway is hardly good.Even slide movement from the closed position won't always give equal flow.Equal vacuum readings gets you close,but there are still variables.

I fit a piece of wire between the lead and plug,with enough wire hanging out to easily short it to the head with a screwdriver.
Short out each cylinder at idle.The other cylinder will fire approximately 5 times before it stops.Both cylinders should be equal.Adjust the idle stops to get it equal.

Short out one cylinder at 3000 rpm,and see what the rpm drops to.Adjust cables to get equal results on each cylinder.

Try this after you think you have it perfect by other means.You might be surprised.
 
1 - 20 of 21 Posts
Top