Britbike forum Forums British motorcycles in general The competition forum Carbs, decisions, decisions.

This is interesting I was riding the 883 and the Firebird today and noticing how they compare. The Firebird except for the head and carbs is relatively stock the 883 isn't. The 883 makes around 97hp at the crank, and it's quite responsive after it gets some rpm up and then it kicks in pretty hard. The Firebird is instant and strong. And that's what makes it nice to ride. It's not like it's faster, it just kicks in sooner.

The 34mm carbs on Ben's 734 mean it does as well. So I might borrow it and just see. Side by side roll ons would be the way to compare those 90s because it's hard to compare. The Firebird isn't going to keep up, but that response if it was available on the 883 would really be something. It's fine how it is but it's possible it could give more if it had smaller intake ports flowing the same cfm at higher velocity. It would still breath, you could increase intake speed with smaller pipes but lose breathing and top end.

The 734 is a bit lower geared and when you get the clutch out, touch the throttle it jumps, it's what we hoped to achieve. Both it and the 883 are similar really.

The 734 is pulling hard from around 3,000. I adjusted the pilot jet and mixtures and set the needles as lean as they will go. It runs really well but is around 13.5-1 air/fuel ratio when warm and cruising. It could be 14-1 or more as long as it goes down toward 12.6-1 opened up, where maximum power should be. Easy starting when warm. It seem a lot less thirsty now. But could possibly be better. Having the gauge to see where it is is really good.

Sweet shift on the 4speed with ball ended plunger, hard to say if you'd be looking for a 5speed with such a power spread, you would have to be racing to find that out. Ben's back toward the end of this week so we may be able to get them side by side, but I think it would be who gets the gas on first.

Finally got some different needles. The std set up worked but the mixture visible on the Air/fuel gauge showed where it was wrong. It ended up with needles in the leanest position. But that leaned it just off idle so I fitted bigger pilot jets. Went from 32 to 42 then back to 40s. The new needles taper begins about 5mm earlier and are a little leaner. In the middle clip rich and cruising on 10.5. I tried the two leaner settings. And there is not much difference in that area. But at 1/2 throttle I saw 13.6. And will go back to 35 pilot and richen the needle. The circuits overlap and at 62mph the throttles are barely off idle. And I expect the rich pilot is combining with the earlier start of the needle making it rich.

mark, where do you have the oxygen sensor placed in that machine? i cant locate it in your images.

nvrmd, i just found it under the crankcases. i use one a bit farther up in the header on a street legal machine, but ive never used it on my LSR straight line bike because ive not run a battery yet

im going to test an EI so i should be able to use tbe AF gauge soon

That didn't fix it, but I can see a lean spot using 32 pilots and the 2nd leanest clip, but it's surrounded by rich. You would not know from riding but it's using excess fuel. Starts easy. Slide cut away I guess could be increased, the richness is in that area. And filing them is about the only option.

Moved the cutaway a little but didn't do much. I'll use a cutter next and make sure to lift it in near the needle, and fit a bigger pilot. It's running 10.5 unless it goes on the pilot area. I saw it on 13 then 14-1 coming home then 15 so turned it to reserve

The thing makes quite a bit of pull on the jetting, reducing the pull on the needle can bring some leanness in there. The cutaway effects idle to half throttle the main area being 1/4 throttle and that area is rich, then further on the needle it leans a bit. The needle effects 1/8-3/4.

The target is 13 cruising and 12.5 opened up.

What I can say about the meter; it tells you which way your out. A miss and stumble can be either way, but it runs fine on 10-1 or 15-1 any more or less it doesn't run right. It pulls fine from 2,500, top has plenty of get up from around 3,200 for overtaking. Then you feel like changing up, but the rear sprocket is 38t. It needs to cruise around 13-1 to get some decent mpg. 10.5 isn't cutting it.

This video is about the Firebird. The engine is a pretty standard 1970 A65 654. What it has is a roller conversion, 9-1 pistons, std cam etc except for a ported Thunderbolt head and twin 34mm carbs. These make it pull strongly from 2,000rpm with no flat spots.

They work but the carbs are not quite right, I need to weld a bung in a header pipe and fit an A/fuel gauge. Then I can see where it's rich or lean and fix it.

It reminds me of the noise the twins make on the video 99.9 where the twins rev up down the hill at the IOM. I finally bought some proper Firebird std mufflers.
It sounds pretty sharp with this camera.

This would need to handle better and be geared much higher for the IOM. Plus have a decent capacity fuel tank. I've ordered some internals for the forks which should improve it, and the gearing is tall enough for the road.

The power characteristics of this make close ratios fairly unnecessary, if gearing was a lot taller they would become more useful. I would not put a big bore kit on this because more power isn't something you could use. Maybe racing with taller gears and handling sorted.

Before I polished the H section.


The main difference with this is this head.



And interesting to-day I decided to test a std 68-701 head I just got from Tom. Generally they flow around 109cfm. I used to calibrate from a head until I ported them all. Then I used a Helgesen plate and worked from it. And now I don't know which is more accurate. This is an older head with smaller port runners but still the 40.5mm valve and the runner size doesn't make much difference because it's not where the limitation is at this point. Calibrating from the plate would mean it flows 98cfm @28"w. Generally they are 109, a '70 thunderbolt port without manifold is 108 calibrating from a head that had been tested on a proper bench. That head is now flowing 181.5 or 195.3 depending what I calibrate from. It doesn't really matter I just need to see when it's better or worse really, and it's better to find out the ported ones are probably better than I thought, not worse.

I also lashed out on some std grips, they are big and fat made to reduce the vibration you feel and seem to sort of work.

The outfit's new head is probably pretty good calibrated from the 68-701 which I doubt is only 98cfm and closer to 109.



This sounds different to the 90s.

Jeez that just reminds me that I have not ridden my bike for a couple of weeks! Gotta go man.

Hi Mark

Look up "Grip Puppies" these make a big difference to bar vibration.

regards

John

Have a similar thing on my bike, several makes are around, 'pussy grips'
is another one.
Best thing i did with my one as the left side was a lot worse is i put
a 14mm bolt about 6 inches long wrapped in rubber tape into the end.
Stopped the tingling at high revs.
The other thing which was pointed out to me some time ago was
'Ride the bloody thing sensibly you idiot' but i choose to ignore that.

Fuel taps came today, so fitted the big tank and finished wiring in the A/F gauge which works great. Its actually pretty close till about 1/2 throttle then it gets lean. I couldn't get big jets from China. Their biggest were 140s, I got a pair of 150s $$ so it would actually run wide open. It almost did on 140s. Anyway Ben had some bigger ones. I need it 12-12.5 not 15-1+.

This thing is now very enjoyable it's smooth enough to cruise at a decent speed.

I ordered some red and yellow stickers so it would be like the production race Spitfires with polished tanks, but stuck these on until the new ones are done. If it pulls harder with the correct mains it will really need the handling looked at. I tried aligning the wheels more exact but it's so dodgey because it needs doing from where the front tyre is on the ground because the wheel leans. It kind of slow wriggles back and forth under hard acceleration. I cannot really find anything that should make it do this.

Been riding around a bit this afternoon with the grandson who got his Ls on Friday. I think he's starting to understand he can jump on his bike and just go where he wants.





That wide band sensor needs me to be careful not to bump it too hard, It just sticks out of the bird cage.

Sounds / looks great Mark, re handling, check your wheel balance, mine used to do something similar before I balanced the rear wheel.

Mark, Your slight wiggle under acceleration may be from the rear Shocks. My 750cc LSR bike had a wobble under acceleration which was cured changing the rear Shocks to Hagons from the cheaper versions. Balancing may be worthwhile too.

It's sort of like it rocks from side to side it does it on bumps but it should just ride over them. I have aimed the wheels the same direction now, I think. I'll check those Shocks. I think I've seen old drag bikes with struts, may be why.

I can try the springs in the softest position and see what that does, I had them jacked up for the granddaughter on the back, and just left them. The damping may not match that much spring. I could also take the springs off and see what the damping is like.

I'll check that balance as well. The bearings feel ok, and that's usually noticeable at slow speed.

If you go to the lightest spring and the wobble gets worse then the Shocks are not matched. Damping is causing more twisting of the swingarm.
Without a shock dyno one possible way to check the Shocks is to rigidly mount them vertically spaced apart with a bar pivoted through the top eyes. Hang a weight in the middle of the bar and see if the bar stays horizontal. You might need a fairly heavy weight, maybe 10 Kg or more, to overcome frictions.
Of course, not much you can do with fixed shock damping other than swap them out for another closer matched pair.

I've checked over and adjusted a few things and it seems much better. Put 165 mains in and it looks better except the clutch again. Dropped the needles and have that same problem with the needle. It goes rich then leans off.

Could you be looking at only half the problem? the handling on my Lightning improved a lot when I rebuilt the front end on my 68' I even made my own damper rods to close tollerences and the handling improved a lot. I did used to wobble at speed - a bit like "fish tailing" but felt through the whole bike"... You would also try another 10ml of oil in the front forks.

Nice looking bike Mark. Love the tank. I had to put the Spitfire tank back on mine as the Small Hornet tank did not give much range and since They are fiberglass I need to use non ethanol. Usually LL100.

I like a bit of color on the tank, this is just a papercut out till I get some made. The colors should be a little brighter.



This is like the 1967 production racers with red and yellow stickers, except for alloy rims and the pipes. But really these pipes are the business.

I have the 2 gal. Alloy tank on my 68 Firebird. Lot of work to do to get a shine like on your tank . Very nice.

Finally the 4 stroke needles made in Italy arrived from Germany yesterday. So I've been testing them today on the Firebird. The outside of the air/fuel gauge has leds; green for power range, orange for cruising and red for too lean by the look. Orange goes over 15-1 I think. Anyway. It can see 15-1 and 17-1 when cold with the chokes off, but that comes down to 14.8 and around there at 2,000 in town in 3rd. If I open the throttle it goes into green and 12.5 and around that and pulls. At 4,000 on light throttle it's 13.6 or so opening at all it's in 12s. The 150 mains were quite lean, so with 165s it's in the green and pushes harder. It also pushes harder at 1/2 throttle. When the throttle is shut it reads way lean and lights up red leds. But it idles around 14. something.

If I use full throttle to get to speed and then cruise it is a little leaner and it either puts the fuel level down a bit. Or needs both taps on. Driving off with the fuel tap turned off and I'm thinking wow why so lean, oh yea. So it's early warning.

The only thing is the clutch isn't happy, maybe the plates are not flat or something, it has extras and super heavy springs. But they don't do it. But it's carbureting really nice. The gauge takes the guessing out, or most of it.

I don't know how the power jet works, if that's the richness when opening the throttle. I could find a little stumble before, but not now.



This tells you what the mixture settings do. So being air cooled, when putting load on, I want it well in that power zone.