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Twin Carb T100C Valve Timing #781846 08/18/19 2:29 pm
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BenM Offline OP
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I'm in the middle of tearing down the engine on my 1968(ish) T100C, which someone long ago changed to have twin carbs. Last night I pulled off the timing cover and saw that the cam gears were set up for what the shop manual says is the single carb valve timing (the crankshaft gear's dots line up with the dots on the camshaft gears). The manual says that for twin carb models, the crankshaft dots should line up with the line marks on the cam gears. That leaves me with a couple questions:

1. What is the reason for changing the valve timing if there are two carbs vs. one carb?
2. Since I have two carbs, should I reposition the camshaft gears, or will it hurt to keep it set up the way it is?

From what I understand, the bike hasn't run since before I was born, so I have no idea why it sat or how it ran previously.

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Re: Twin Carb T100C Valve Timing [Re: BenM] #781868 08/18/19 5:21 pm
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Gordo in Comox Online Content
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With dual carbs the two cylinders are basically acting independently from a tuning point of view. With a single carb they are sharing the inlet tract and it must be that there is an advantage to slightly changing the valve timing. If you look at the key slot in relation to the dots and lines you will see that it is a small difference between the two setups.

Did you confirm that the camshaft keys are actually in the slot opposite the dot? Did the engine actually run as set up?

Gordo


The roadside repairs make for the best post ride stories.
Re: Twin Carb T100C Valve Timing [Re: BenM] #781878 08/18/19 6:39 pm
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I suspect that the difference in cam timing between the single and twin carb models was primarily due to a change of exhaust camshaft:



E10043 Inlet camshaft

E10047 Exhaust camshaft TIOOT, TIOOR

E10046 Exhaust camshaft TIOOS, TIOOC, T90


You will only know if the camshaft was changed with the head by detailed measurements of lift, or by removing it to see numbers cast/stamped on it.

My guess is the cam wasn't changed, so the builder left the timing as original.

I'd suggest running as is, I'm sure it would run fine on that setting with either ex camshaft.
If you're curious enough, it is easy to reset the timing gears to the daytona setting just to see what difference it makes. You will have to retime the ignition if you do this, no big shakes.

Re: Twin Carb T100C Valve Timing [Re: koan58] #781890 08/18/19 9:05 pm
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BenM Offline OP
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Originally Posted by Gordo in Comox
With dual carbs the two cylinders are basically acting independently from a tuning point of view. With a single carb they are sharing the inlet tract and it must be that there is an advantage to slightly changing the valve timing. If you look at the key slot in relation to the dots and lines you will see that it is a small difference between the two setups.

Did you confirm that the camshaft keys are actually in the slot opposite the dot? Did the engine actually run as set up?

Gordo


I'll see where the keys are once I get the nuts off the ends of the camshafts. I hope they are installed right, but wouldn't be shocked they are keyed wrong. As for the engine, I honestly don't know if it ran the way it is. Given the heavy coating of old oil caked on the outside of the cylinders and cases, I'd say it ran, but not very well. My dad acquired it in 1990, and I know he's never seen it run.

Originally Posted by koan58
I suspect that the difference in cam timing between the single and twin carb models was primarily due to a change of exhaust camshaft:



E10043 Inlet camshaft

E10047 Exhaust camshaft TIOOT, TIOOR

E10046 Exhaust camshaft TIOOS, TIOOC, T90


You will only know if the camshaft was changed with the head by detailed measurements of lift, or by removing it to see numbers cast/stamped on it.

My guess is the cam wasn't changed, so the builder left the timing as original.

I'd suggest running as is, I'm sure it would run fine on that setting with either ex camshaft.
If you're curious enough, it is easy to reset the timing gears to the daytona setting just to see what difference it makes. You will have to retime the ignition if you do this, no big shakes.


Thanks for the info, Koan! I'll probably leave the valve timing as it is, assuming the keys are installed right, as Gordo mentioned.

Re: Twin Carb T100C Valve Timing [Re: BenM] #781996 08/20/19 2:47 am
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Wilfred Offline
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My 67 Daytona engine has Daytona cams, 1 1/4 followers and is timed as per Daytona timing and I run a 930 single carb on it and am quite happy with the way it runs. I tried two 626 carbs and went back to the single 930. The shop where I was buying parts at the time recommended ( after I mentioned that the bike came to me with TWO 930's on it ) using one 930. Been that way forever.

On the 68 Daytona engine that is in my ISDT replica project, I am going with C cams and followers, one 626 carb and will time it as a T100C. It will be interesting to see how that works out although I will probably use a 16 tooth gearbox sprocket and a 50 tooth on the rear wheel unless I use a wide ratio box and then I will go with standard 18 tooth gearbox and a 46 rear wheel sprocket to give me the standard 4th gear ratio when on the road. Hope to get the ISDT bike finished soon so will start with one system and if not happy try another. Isn't that half the fun?

Cheers, Wilf


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Re: Twin Carb T100C Valve Timing [Re: BenM] #782019 08/20/19 8:43 am
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kommando Online Content
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The OIF TR5T is effectively a single carbed Daytona.

Re: Twin Carb T100C Valve Timing [Re: BenM] #782024 08/20/19 11:43 am
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Andytheflyer Offline
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I've just re-built a 72 T100R Daytona with twin 926s that I acquired in bits and has been in bits since 1982.

I had a local restorer have a look at the major engine parts when they vapour blasted it, and I asked them to take otf the camshafts, check the bushes and replace if necessary. They said that the bushes were fine and they refitted the cams and their timing wheels.

When I was setting up the valve timing and fitting the crankshaft pinion and idler, the engine would not turn over all the way. Spotted that the inlet valves were open at TDC on the compression stroke. Even on the exhaust stroke they should not be open. Did a bit of research (but can't remember where now!) and discovered that the dots and lines on the cam pinions are for different ages of T100R - it was set up on the dots, but it needs the lines. Bought the correct puller/fitter and moved the pinions to the line marks. All OK.

So, as I understand it, the valve timing marks to be used depend upon the engine number. Not sure why it would be affected by the number of carbs though.

And the engine fired up 2nd kick and runs just fine so I must have got something right.


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