Can someone tell me why a starter motor would be sluggish for the first start of the day but if once the van is started then switched off and restarted the starter spins at a normal speed? Is the cold weather likely to be a contributing factor? Battery is fully charged.
Your battery may seem fully charged, but may not be up to the job under load. Got another one you can try?
The starter on my Beetle has always struggled in the morning, and has now got to the point where it won’t spin fast enough to start, even with a jump from a running car. . I would say your starter is probably on the way out. (Assuming a known good battery makes no difference)
Cold oil, stiffer engine as a result, battery output lower because of the cold, also the air is a bit denser so compression is harder, grotty earth strap bolts.. . Probably an old battery too. A fully charged well sulphated battery can be down to as little as 1/10 of its original capacity and still read a good voltage until you crank. If the battery voltage goes below 10 volts while cranking, you need more power to turn the engine from a better battery. However, with the prices of even GSF starters now being astronomical compared with a few years back, the Powerlite geared ones are actually cheaper than the GSF at full price, although GSF are running a 60% discount at the moment. But the JK cheapo "stock" is £132, or £185 for a Hella brand remanufactured.. more reasonable but more than the £75 I paid GSF back in about 2016 for my starter motor...
Cold oil for the win and maybe fuel still being pumped upto the carb/s. But otherwise you'll need to go through everything as Mike says.
When was the bushing last changed. If there is “any” wear on the solenoid it can “pole” (pull off centre) and bind on the bush.
As long as it is a voltage drop problem and not a physical/mechanical problem. If it is, then, if you can, resolving the issue is a better option. If all the previous fail, then a hot start relay is a good option.
Mines same with a new Bosch battery, I put it down to the rather OTT 170psi cylinder pressure + all those other factors.
I had fitted a hot start relay as it also prolongs the life of the starter switch. And the other day while the engine was out I took the back off the GSF recon Bosch solenoid on the starter - needs an agricultural soldering iron or a small blow torch plus soldering iron as the solenoid coil is soldered in. And cleaned up the slightly burnt contactor in there with sandpaper. I also have new earth straps. And a newish 45AH starter battery. No trouble cranking a 1641 with 20W 50 syrup at -3C..
Hot start relay fitted, new starter bush and earth stap fitted fairly recently and the battery is only 6 months old too. 12.72v at the starter dropping to 11.3v while cranking. It spun over a bit faster this morning and its colder outside so i'm thinking lack of use could be the cause. Its my spare starter (a 0.8kw unbranded one) thats been sat about in the shed after being used on my engine test stand a couple of years back. The one that was on the van (a genuine 1.0kw Bosch one) lasted 9 years since it was last rebuilt so its going to be getting checked out and possibly rebuilt again. I've heard you're lucky to get a year or two out of the new cheap chinese ones if you're lucky.
