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| Thoughts on timing marks | Rate Topic |
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| Posted: 03-09-2023 02:20 am |
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1st Post |
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JH12947 Member
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I have this engine running on a stand and it starts immediately and idles well, but is does backfire through the exhaust when the RPMs increase. When I changed the timing belt I ended up with a very slight misalignment in the can pulley marks when the crank pulley is at TDC as can be seen in the attached photo. Having never seen this or any 907 engine run, I'm not sure whether exact alignment is to be expected, or whether some small misalignment like I have is normal. Moving the belt by a single tooth results in far more adjustment than I would need, but perhaps some amount of fiddling could get there. Thoughts? Attachment: Cam Marks (4).JPG (Downloaded 98 times)
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| Posted: 03-10-2023 06:04 am |
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2nd Post |
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Esprit2 Member
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With the stock, non-adjustable cam pulleys, you can only index the pulley on the timing belt in whole tooth pitch increments. 1 Tooth Pitch = 1 Tooth Width + 1 Gap Width... or, from any point on one tooth to the exact same point on the next tooth. You're off by less than a tooth width, not to mention gap width. Adjusting the pulley on the belt one full tooth pitch in either direction will only make matters worse. No adjustment you can make with non-adjustable pulleys will bring the timing marks into perfect alignment. All that presumes that the crank is accurately set to TDC. if it's not, then 'SET IT TO TDC !!' and re-evaluate the cam timing. Set the crank to #1 Cylinder at TDC by rotating the crank in a clockwise direction, and stopping cleanly at TDC without over-shooting or backing up. If necessary, instead of rotating the crank two full revolutions and back to TDC, it's acceptable to back-up counter-clockwise to ~30 BTDC, then reverse and rotate clockwise back to TDC... stopping WITHOUT over-shooting or backing-up and introducing any slack into the belt. Pull to a clean stop with #1 at TDC. Anything that affects the distance between the crankshaft and camshaft centerlines will either introduce more slack out of the belt loop, or pull belt length back out of the loop. Things like dimension tolerances on all of the parts involved, a milled block deck or cylinder head, or a thicker or thinner head gasket (the modern composite head gasket's compressed thickness is 0.5mm ~ 0.020" thicker than that of the original steel-asbestos-steel head gasket). Total tolerance/ error stack-up will cause a small mis-alignment between the two cam timing marks, but usually there is nothing you can do about it short of installing adjustable cam pulleys. Which raises the degree-of-difficulty bar considerably. For street driving, having the cam timing off by some fraction of a tooth width is not worth losing sleep over. Regards, Tim Engel Attachment: 0 - 907 Cam Marks on JH.jpg (Downloaded 89 times) Last edited on 03-10-2023 06:16 am by Esprit2 |
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| Posted: 03-10-2023 06:19 pm |
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3rd Post |
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Darth V8R Member 
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A misadjustment that small will not result in what you are experiencing. Instead, look at ignition timing and spark plug wire routing (e.g. two plug wires swapped). Also, be certain that the during installation the distributor was installed in the correct orientation, and that the bob weights are not sticking so that the timing is able to advance. Make sure you have the correct type of distributor rotor so that you do not get cross firing of the plugs. Check your distributor cap for carbon tacking and replace if needed. If you are running stock ignition, try a different condenser as a bad one will cause erratic spark at higher RPMs. Lastly, run the engine under low light conditions and check for visible high voltage arcing from your plug wires. In my experience, 50% of ignition issues are due to non-stock ignition setups that are improperly installed by the owner, so if you are running non-stock ignition as a last resort switch back to stock ignition to see if that corrects the problem. I had to do that on my Dolomite Sprint motor as the electronic ignition was mis-behaving. Vance
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| Posted: 03-10-2023 07:17 pm |
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4th Post |
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JH12947 Member
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Thanks for confirming my suspicions on the timing marks. I can move on to look at other possible causes. FWIW, I have left the rest of the ignition as I found it on the assumption that it ran with the installed components. New plug wires and coil, but stock cap, rotor and points. All look to be decent. The advance is working at the rpm increases. i have the vacuum retard capped at the moment. The car has 45 DHLAs on it and in looking at the recommendations in the forum it seems over jetted on the cold start and pump jetting. I think it may be overfuelling so that's likely my next stop.
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| Posted: 03-11-2023 12:00 am |
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5th Post |
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Esprit2 Member
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JH12947 wrote:(Snip)... The car has 45 DHLAs on it and in looking at the recommendations in the forum it seems over jetted on the cold start and pump jetting. I think it may be overfuelling so that's likely my next stop. What size chokes are in your 45DHLAs? Are the carbs properly mounted with 'Soft Mounts', and without over-tightening and crushing the O-rings? Are the carbs properly 'Balanced'. And, "Well, I tried" isn't the same as "Yes". Have you accurately balanced the carbs? The engine will run like crap when the carbs are out of balance. On each carb, under one of the screws that secures the Top Cover, there should be an oval, aluminum tag with a 4-digit number and a letter (A or P) stamped on it. What are your carbs' two Tag Numbers? What is the current jetting set-up in your carbs? Jensen-Healey . . . . . . . . . . . . . . . . . . . NON- . . . . EURO . . . . . . . . . . . . . . . . Emissions . . Emissions. . . Yours? Throat. . . . . . . . . . . . . . . 40. . . . . . . 40E . . . . ___ (45, or 45E ?) Choke . . . . . . . . . . . . . . . 35. . . . . . . 35. . . . . 36 Aux. Venturi. . . . . . . . . . . 8011.1 . . . 8011.1 . . 8011.1 Main Jet . . . . . . . . . . . . . 130. . . . . . 130. . . . . 158 Main Air Corrector . . . . . . 160. . . . . . 160. . . . . 230 Main Emulsion Tube . . . 7772.5 . . . . 7772.5 . . . 7772.5 Idle Jet (4 Spd). . . . . . . . . 50 . . . . . . 55L . . . . 58 Idle Jet (5 Spd). . . . . . . . . 50 . . . . . . 52L . . . . ___ Idle Air Corrector . . . . . . 7850.1 . . . 7850.5 . . . 7850.9 Pump Jet . . . . . . . . . . . . . 45 . . . . . . 45 . . . . . 60 Starter Jet . . . . . . . . . . . . 70 . . . . . . 70 . . . . . 95 Starter Emulsion Tube. . . 7482.1 . . . 7482.1 . . . 7482.3 Float Weight . (gm) . . . . . 8.5 . . . . . . 8.5. . . . . ___ Float Height. . (mm) . . 16.5-17.0. . 16.5-17.0 . . ___ Needle Valve . . . . . . . 7180-15. . . 7180-15. . . . ___ Mixture Screw . . . . . . 3 1/2 to 3 3/4 Turns Out . ___ Note: Standard jetting specs presume Sea Level (ie, standard atmospheric pressure). The higher your location is from Sea Level, the less air will be ingested on each stroke, and the richer the mixture will become... to the point of chugging and blowing black smoke. The factory setting should work up to about 2500' ASL, depending upon how fussy you are. Over that, plan on re-jetting the carbs for your altitude. Regards, Tim Engel Last edited on 04-22-2023 04:01 am by Esprit2 |
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| Posted: 03-11-2023 01:43 am |
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6th Post |
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JH12947 Member
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Thanks Tim, I have a 4 column manometer arriving tomorrow so balancing the carbs will be my next move. I have only single DCOEs on other cars so the twins is a new wrinkle. The mounts have new o-rings and are tightened to spec. Tags on these carbs are 5325 and 5326 (looks like "R" but could be "P" I suppose). I'm 1100' above sea level. The jets it arrived with: In the car DHLA45E Choke 36 Aux. Venturi 8011.1 Main Jet 158 Main Air Correction Jet 230 Slow Running Jet 58 Slow Running Air Jet 7850.9 Pump Jet 60 Starter Jet 95 Main Emulsion Tube 7772.5 Starter Emulsion Tube 7482.3
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| Posted: 03-11-2023 04:56 am |
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7th Post |
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Esprit2 Member
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5325 P, and 5326A ?? If so, then those carbs were originally for the Sunbeam-Talbot Lotus rally cars... the 'street' homologation version. The engine was the 2.2 Litre 911. In a recent message somewhere, it was written that Judson Manning had rebuilt such a set. Are those the carbs we're talking about? Regards, Tim Engel Last edited on 03-11-2023 07:04 am by Esprit2 |
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| Posted: 03-11-2023 07:15 am |
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8th Post |
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JH12947 Member
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No. I pulled the tags had a closer look. They are quite definitely R5325 and R5326. No A or P. Perhaps "R"econditioned? In any case, they are spec'd exactly like the Talbot 2.2 so it's a good assumption. Jets seem ok except the pump and idle circuit. Engine will idle fine with the mixture screws completely seated and backing them out has little impact. I'm not sure that the car every ran well this way. The dellortos were installed when I dragged it home, but the Stromberg setup was in a box. I have a feeling the PO may have just stuck them on as is. They were full of varnish so had been run for a while but need to get it sorted now. Attachment: IMG_7941 (3).JPG (Downloaded 74 times) Last edited on 03-11-2023 05:24 pm by JH12947 |
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| Posted: 03-12-2023 05:47 am |
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9th Post |
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Esprit2 Member
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JH12947 wrote:No. I pulled the tags had a closer look. |
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| Posted: 03-12-2023 02:12 pm |
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10th Post |
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JH12947 Member
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Esprit2 wrote: Is your engine a basically stock 907... ie, 2.0 Litres? The 911 is a 2.2 Litre, and carbs jetted properly for a 911 will probably be too rich for a smaller, 2.0 Litre 907. This I don't know for sure. Based on the low mileage (40,000 mile), and the generally basic nature of the engine, (stock ignition, stock exhaust) I'm going to assume stock, but of course there is the chance it's been warmed over. Esprit2 That's not good. The Idle Mixture Screws should end up about 3 1/2 to 3 3/4 turns out. Less than that (and fully seated IS LESS than that) indicates the Idle Circuit is too rich, and you're trying to compensate by adjusting the Idle Mixture Screws too lean. Yes, I'm going to lean this circuit out as a first step. Thanks for your replies. Corey
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| Posted: 04-24-2024 02:38 am |
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11th Post |
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Esprit2 Member
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Progress Report ??
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| Posted: 04-24-2024 04:00 am |
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12th Post |
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JH12947 Member
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Esprit2 wrote:Progress Report ?? I'm still waiting for delivery of the spec 5 main and idle jets before I have another go at getting it up and running. I also had to get the flywheel machined so had the tranny out as well. That job is done. I pulled the intake off and resealed that, as well as reset the choke plates, generally cleaned it all again and set the fuel level at 25mm as suggested. When setting the fuel level I found that one of the carbs was puking fuel into the barrels. I swapped out the needle valves which seems to have fixed that. When the jets show up I'll start the balancing again and have more progress to report. Thanks for the help so far.
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